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.
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
In the case of multiple cuts in series, place a stop against which the piece to be cut is beaten. You can make cuts in series without repeating the measurement and without marking the cut on the piece.