I'm trying to homebrew an item that increases ability scores beyond 20 and I'm not sure how, can someone please help? 

You need two modifiers:

• Modifier: Stacking Bonus
• Subtype: [ability score]
• Fixed Value: [a number between 1 and 10]

That will give them a bonus capable of going above 20.

• Modifier: Bonus
• Subtype: Ability Score Maximum
• Ability Score: [ability]
• Fixed Value: [a number between 1 and 10]

That will actually raise the maximum, it cannot go above 30.

Thank you very much

I hope it isn't an issue that I comment in a thread from half a year ago. I just have to say thank you, IamSposta. I create quite a bit of homebrew. Every time I run into something I can't figure out I search for the answer, and most of the time without fail I'll find you in the thread giving the answer I was looking for in a format so simple that anyone could follow along. You're the best, truly.

As usual, I’m just happy to help.

Perhaps you'll also know if it's possible to create a feat (like the top tier boons from MOoT) that gives players a choice between which score to increase by 2 (as well as the maximum). :)