Hello! How do you build or convert creatures?
There are a couple of ways.
The easiest is just using stats right out of the Old School Essentials Monster Book (also available online as a free SRD) or the original Monster Manual.
I tend to use those exclusively for most monsters, though I often cross-reference the 5e MM to simplify some of the goofier old-school monster-specific rules.
If I want a cool boss fight, I start pulling special abilities from the 4e Monster Manual, and give them “Villain Actions” (see matt Colville’s “Action Oriented Monsters” on YouTube).