To make a really straight cut with a hand-held circular saw we will need a guide.

As a guide we can use any material available that is sufficiently straight. An aluminum corner piece, a square panel, a strip.

This reference will be fixed in position considering the distance of the blade from the edge of the circular saw cutting plane.

Alignment

Consider that the distance between the blade and the edge of the circular that will rest on the guide is 10cm.

We will then position the guide 10cm from the edge of the cut we want to make.

Fix the piece and the guide in a solid way, with some clamps. Now it can be cut