I honestly don't know if I am alone here, but I oftentimes find the Monster Manual (and, by extention, the SRD) leaving much to be desired when I'm looking for an NPC statblock for my players to fight with, because sometimes, I want my players to deal with stuff that you can't just find there, or that you can, but the stat block is always either a push-over or a TPK waiting to happen. What if I want the players to battle a band of elf paladins? or a Tiefling Druid? how would I translate NPC power to CR?

If I want to design monsters and NPCs with combat in mind, I usually have to rely on what I can find in the Dungeon Master's guide - but the DMG doesn't say anything about calculating CR using a monster's spells (besides damage, AC, healing and DCs), it's ability to inflict particularly dangerous conditions, buffing or controlling, summoning, Lair actions, etc. which, as a matter of fact, can play a critical role in how tough an encounter can really be. How can I properly assign CR if I don't know how to assess these sorts of things?

So how do you go about it? do you build your own monsters? do you have any tips or suggestions on the matter that could make building your own encounters from scratch that much easier?

Thanks!

I make the stat blocks of my monsters somewhat reflect that of my players, for an easy encounter the monsters will have lower stats, for a medium encounter the monsters will have similar stats, for a difficult the monsters will have higher stats.

When I create stat blocks for homebrew monsters or opponents, I often base them on one already contained in one of the available books. I find that this makes things a bit easier and cuts down on the math. With a few slight changes and some different color copy, many of the things available can be made to feel unique and diffierent for players.

I usually copy an NPC or monster from any of the books (mostly ToB and VGtM) and tweek them to what I want.

I also keep my old character sheets, and sometimes make copies of my friends. Old characters make great NPCs.

I usually modify stats of existing material to fit my needs. I also find that sometimes mixing different rating monsters in one encounter allows for enough of a challenge to players that I don't have to modify them much at times.  

For NPC's I usually. Yield them like characters and have them be a level or two bellow my players stat wise. If they stick around for a while or return to the players side I level them up or give them atleast the spell access they would get as The players lvl.

really breaks down to me that sometimes smugging details for the sake of the story and game flow.fir example I have had my players drag a NPC into a monster Lair it had no reason to be there. I had to come up with that players stats on the fly, person was a magic user so rolled up hp with wizard hit points die, quickly built stats using point buy, and listed spell slots according to players lvl. Fleshed out details more after session.   

I've always designed my own monsters when I want something not already in the books. I'm glad that 5th edition made it simple again (adding what I want and assessing the results, rather than following a predetermined formula to the letter).

Mostly, though, I find plenty of use for the monsters in the books, so my designs are most often either a small tweak (I made a different age and color of dracolich than is in the monster manual example, and made a dragon that is also a mummy, for example), or is a conversion of something from a prior edition that hasn't made the 5th edition books yet.