How to make a long straight cut in a board at an angle?
Sawhorses with a board on top to form a makeshift table, then your green board on top, and a straight aluminum ruler (up to 3m, in most hardware stores) fastened with clamps to the green board parallel to the cut with an offset as big as your circular saw is wide? Or is your circular saw fastened to a saw-table? 

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image: amazon.com

Alternately use a very straight board clamped to the work as a guide for the saw.