Here are the things you need for measuring ann cutting the wood.
First measure the wood from the side to check if it is 90 degrees.
After you measure it from the side then measure it from the front with the carpenter square.
Also don't forget the long side and make sure it is 90 degrees.
Measure the piece of wood with the measure tape to and mark the place where you want to cut it and make lines around all the faces of the wood with a pencil.
Before cutting the wood you should have some thing like a C Clamp to hold the wood with and make sure you put the wood in a place you can have a good angle for cutting it.
Now we come to the steps where we will start cutting. You need a saw for this now and try to be careful with it and hold it straight while cutting.
When you feel like that you are are off the track you can make it straight by cutting it from the side.
After you have finished cutting check the angles of the wood again and to see that you have cut right.
I didn't cut it right and don't worry if you cut it like this because no one is perfect and try to give it another try.
Now after trying again my wood is cut straight.