How to make a small piece of wood smaller with limited tools
Your circular saw will do the job, but you should make a jig to guide the saw instead of trying to follow a pencil line. You will need a couple of pieces of straight lumber a little longer than the cut you want to make. Many books and articles say to use scrap wood but I sometimes use good lumber destined for use later. 

The technique is to cobble together an assembly with an inside corner that the saw's foot plate can be pressed against, with the piece to be cut positioned in the path of the blade. Sometimes you can clamp the assembly together but I find that the clamps get in the way so I usually use small nails. 

The shape of the jig will depend on what material you have on hand to make it. Here is a suggestion: 



Please forgive the crude cross section but it should convey the main idea.