D&D 3E/3.5 Converting From 3e-ish to 5e?

[Flamestrike]

This. My rule of thumb is: if it's a d20 roll or DC, change it to ability bonus + proficiency bonus. If it's an AC, look at the PHB armor table and pick one that seems comparable. If it's anything else, you can usually leave it.

Mine is, 'use DC 15'.

Unless its obviously supposed to be super hard or something in which case its 20.

Super easy and its 10 (or even, a PC thats proficient; succeeds).