Start by picking a piece of wood that suits your need.
Next use a carpenter's square to text whether your piece is 90 degrees. If you have a commercial cut its better and this step isn't necessary.
Next measure the amount of wood you would like to cut off. You can use either the carpenter's square or a measuring tape. Mark this by using a "v" shape.
Draw parallel lines going all the way down 3 sides of the wood to ensure that you have a straight and clean cut.
I used a Japanese hand saw as these are easier to cut with but you can use whatever saw you wish.
Clamp your wood onto the table. As i am left handed I do this in a way that is comfortable for my left hand. For a right hander you can stand the other side of the wood.
Make sure that you cut all the way down and the saw can not be at an angle otherwise you will have a wonky piece of wood.
Start your first cut by pushing on the saw. The first cut is vital and is important to get it right.
Finally the wood should just fall off and if you followed the steps the wood should be straight at a 90 degree angle.