Here's is one of those basic carpenter tricks that most carpenters learn as apprentices.

As Carpenters we all need shims. You can buy them pre cut, buy a bundle of cedar shingles and rip them to width yourself or you can make them from scrap 2×4 or 2×6.

The thing about shims is its better when they are identical in size.

The best way to make identical shims is with a shim cutting jig.

This video shows you how it's done.