I have an old garage building that we moved and it's out of square. What's the best way to square it. It's 12 x 16 and is sitting on blocks in the corners One side is leaning forward
"Leaning" any way is very bad and it should've just been torn-down. But, since you want to keep it. For a vertical lean you need to install permanent Diagonal Bracing (on the inside) once you temporarily brace it (on the outside, on the building or away from the building) once pushed and or pulled to "plumb" (vertically level) or square.
Instead of diagonal bracing, today's methods use plywood as an exterior skin to keep buildings square. But, Diagonal Bracing runs from the top of a wall's outer corner down to the bottom of the wall as far back as the wall is tall. Typically at a 45-degree angle, so your pieces between studs are easier 45-degree cuts. This " \ " is what your braces and line of braces should look like.