So for anyone who doesn't want to click any links, but wants to learn how to become a lich and a good bit about them, here's the process:

Gain the knowledge to become one (sidenote, this knowledge is near impossible, emphasis on near, to get without somebody that already knows it and is only kept by the most powerful beings, which only give it out to those that have proven themselves incredibly powerful, knowledgeable, and worthy); you will learn written incantations and potion ingredients, with the incantations binding your soul and the potion disrupting your life itself, as well as preparing you to go into it.

Gather the potion's ingredients (it is essential that this be done before making the phylactery, and all ingredients must be gathered within 60 days of each other, and DO NOT add them to the container as you go along); this can be basically anything, but if you want a free example, use the video from AJ in the sources at the bottom. They are usually incredibly hard to get, require massive amounts of planning, and are incredibly immoral. They can also vary, with the idea simply being to completely put your natural life forces off balance. 

Mix the potion; all ingredients must be added at one time, under a full moon, and assumedly in a place shrouded in dark magic, such as a Lich's lair (your own prepared one, perhaps), a graveyard, or a place that oft' practices dark magic, whatever that may be in your campaign.

write the learned encantations on, in most games, whatever you want within the DM's discretion. Once it has been made, it is active, and the potion must be drank within one week after this point.

After the phylactery has been primed, you drink the potion. I'm not sure if it's still a thing, but in an earlier version there was a chance you would outright die instead of the desired effects taking place. Therefore, it would be advised to have someone ready to resurrect you, or have a Wish spell ready, because, and I quote from my source, "they can have some particularly debilitating and devastating effects," whatever that means.

And nothing seems to happen, but so long as you drank the whole potion and did all the steps right, it worked, and you're immortal.

And, finally, there's this special effect with the phylactery. It simply can't be located. Even a god with sensory powers over the entire plane can only detect it if within 100 miles, and everything below that has absolutely no clue, and the Lich can't be compelled to tell the person, even when charmed. those rules are absolute. 

Important things to note that do not pertain to becoming a Lich: you have to consume the souls of other beings, you are almost certainly a master of magic either by the beginning or end of the ordeal, you still have full emotion, and all that humanity fades over time and if you weren't already evil from going through with the horrible things you had to do to become one, you likely eventually will, or you'll just destroy your phylactery because you're done with the planes of reality. You also are almost certainly completely self centered, and someone like a Cleric, Paladin, or any good aligned character would be way more likely to destroy a Lich than even think about becoming one, and you can technically fall dormant if you're starved of souls, but never truly die (unless your phylactery is destroyed, then either nothing happens, your soul is judged and sent to the proper afterlife, or you just fade into nothingness, idk choose what's best for your campaign). You could also become a Demilich, although that's in Xanathar's Guide and I don't have access to it because I'm poor, but they're apparently 3/4 Lich 1/4 undead and nothing but a skull and a phylactery, so look it up if you have the book. And apparently your soul is broadcast to your body from the phylactery like Wifi, but over all the planes I'm pretty sure.

sources: Runesmith's video, AJ Picket's video, and the Wikipedia article I didn't use but rather got off of AJ's video description and thought to be possibly helpful.

Noice!

A sneaky way I got my PCs to waste a PWK and make a lich fight last longer was to use a programmed illusion of the lich (Like in Tomb of Horrors) and then come out swinging after ending the wall of stone spell concealing it.

you are now the final boss for my one-shot

Would a Lich’s ray of frost deal 4d8? Because they’re 18th level casters

Quote from starcanary >>

Liches be Crazy.

 Its been almost 4 years and this still makes me laugh everytime i look for the lich statblock

I never red the requirements of maintaining a phylactery closely before. It must be really expensive to be a lich. Imagine spending 500 gp on casting imprisonment on a commoner, and they roll a natural 20 on their saving throw.

Quote from Flopear >> 

Clone Doesn't save you from old age.

 Sure it does. You just make the clone a younger version of yourself, again and again.

So apparently liches and eldritch liches are enemies which seems cool

Depends on the DM. It's never really described in the Forgotten Realms, but sources for the secret include Orcus, Vecna, and the Tome of Ultimate Evil or whatever it's called.

So, because people are probably checking this stat block fairly often, here's how lair actions work:

"Initiative count 20" just means "the lair action has an initiative of 20."

"Losing initiative ties" means that if someone rolls the same initiative as the lair action, the lair action goes after them, and they go first.

And obviously, this means that all players or creatures that roll an initiative of 20 or higher get to follow regular initiative order.

Finally, the lair action happens every turn that it feasibly can. The only limits are that the same one can't happen twice in a row, it follows initiative order, and you can only choose the ones listed or custom-made

DISCLAIMER:

This DOES NOT mean that the lair action happens after 20 rounds. I don't know why I thought so at first, but it very clearly DOES NOT. Almost all combat lasts an average of 6 rounds, maybe it's 6 turns, I don't remember, but it's over very quickly, and both options would have the lair action collecting dust throughout your whole campaign.

WARNING! Read on, otherwise your soul will be taken, even if you read the word "warning"! Once upon a time there was a man named Duke Hunapon. He was lazy and very indifferent. He always wore a jacket, no matter how warm it was outside. He had an older brother named Michael who always ruled him. One day, Michael was killed, which greatly influenced Duke. He went mad and started murdering people. Soon he got into a fight with someone and was killed. Now he wanders around as a tall skeleton in a red shirt and the same hoodie Duke wore. This skeleton was known as the "Swapfell Papyrus" and will kill you if you don't post it in the 15 comment sections of any website before bedtime. If you fail and wake up with him in your room, your death will be slow and very painful. A girl named Lily Lilupan read it, and he wasn't listening. She was (EEE) and killed in her sleep. If you copy and paste this into 15 comment sections on any website before bedtime, Swapfell Papyrus will make sure you feel safe

RIP Pungo

son of a lich

https://makeaskillcheck.com/5e-how-to-become-a-lich/

Hope that gonna help you in your quest that gonna bring you sadness and madness. 
This is a video that talk about nearly all about it, expect becoming one, so you can use there inspiration to make the lore of your character that success to become a lich.
https://www.youtube.com/watch?v=MDUJIKmvQEM&t=12s

LICHES ARE SPOOKY SHIVEr ME timBeRs I am shAkInG In mY boOTs.

Actually it does

You'd think it'd have chill touch

Quote from Dragonfiremalus >>

I'm curious why anyone would seek to become a lich when immortality is much more easily achieved via the Clone spell.

Legendary resistances, legendary actions, better connection to the soul (ie body cant be separated from the soul, tethering damage to others, freely calling spirits to help you, disrupt life), the ability to kill most things by touching them, permanent true sight, a bunch of condition and damage immunities, and infinite spell slots with your lair action, for free, for eternity. That is why.

yis