I'm looking to make a custom spear that basically is the spell shadow blade. 

• I want to replace its default 1d6 + strength piercing with 2d8 + int psychic. 
  • I have already managed to replace the damage type from piercing to Psychic and switch the modifier from Strength to Psychic using the "Replace Damage Type - Psychic" Modifier. 
  • I have removed the versatile property (this is really more of a throwing spear) using the "Ignore Weapon Property" modifier. 

but for the life of me, I can't figure out how to turn 1d6 into 2d8 points of damage (base damage modification?) 

Can anyone help? 

You can't modify a weapons base damage right now.

I think you can do this by copying the rapier. Change name, Remove finesse. Add thrown. Change damage type to psychic. Add bonus damage of 1d8 psychic. 

Edit:

couldn’t make the int thing work, I think you can customize the to hit and damage modifiers in the character sheet though, but here is a 2d8 psychic weapon called shadow spear https://www.dndbeyond.com/magic-items/1300059-shadow-spear

I also added a new spell and feat that gives you access that spell. So you can add the feat to your character sheet, and then you should see it appear in your attack actions (you need to add both to your home brew collection). This should work the way you want.

cantrip: https://www.dndbeyond.com/spells/372098-spear-of-shadow

feat: https://www.dndbeyond.com/feats/233808-shadow-spear-wielder

Yeah. Modifiers for making weapons that don't use STR or DEX just don't exist yet (things like hexblade and battle Smith are done differently).

Making weapons with non-typical damage dice is also a chore, but I think they told you the best way to do it. It will display weird on character sheet (it will say 1d8 and only when clicked on will you see bonus +1d8).

But stuff like this is planned.

Update: I basically ended up making it a spell, and then made a magic item that just gave the ability to cast the spell at will. It's definitely a workaround but it does appear to be working. 

I'm also trying to make a magical item, one that gives the user the abilities of a Mark of Shadow Elf (minor illusion at will; darkness per long rest; invisibility per short rest)

I managed to get the spells inputted but can not figure out how to make the last two spells have the proper number of times they can cast.... I'm making it from scratch but if someone can point me to the correct item to copy it from I would be most grateful.

Quote from TinyDemonAnima >>

I'm also trying to make a magical item, one that gives the user the abilities of a Mark of Shadow Elf (minor illusion at will; darkness per long rest; invisibility per short rest)

I managed to get the spells inputted but can not figure out how to make the last two spells have the proper number of times they can cast.... I'm making it from scratch but if someone can point me to the correct item to copy it from I would be most grateful.

 You cannot, Magic items don’t work like that. You would either need to make them as separate magic Items, on make them as a feat or something else besides magic items.

The homebrew tools are (essentially) the same tools used to input official content. If you are trying to make something unlike anything that officially exists, you are likely to find you can't.

Drat okay, thanks everyone

If it makes you feel better the homebrew tools are planned to be upgraded in the future (not any time soon probably) and might be more flexible then.

I figured out how to do it with a feat but thank you ^-^