If you do not have a table saw and the run out table big enough for this job and you do not have a track saw you can use a straight edge and clamps.
The straight edge (hear after referred to as SE ) needs to be rigid so it will not flex in the middle, a long level works, but some times it so thick that the motor of the saw may not pass over it, and the clamps can also interfere. Testing is required with your saw.
I have a long SE that is not as rigid as i would like so i have to cut a 2x4 the correct length and clamp it so it reaches the middle of the SE from the side to keep the saw from flexing the SE.
I created a block of wood that is as wide as the distance from the edge of the saw fence to the blade so i can place one side of the block on my mark and the SE up against the block. Then i know that with the fence against the SE the blade will hit the mark.
For 45 deg cut with circular saw a sharp blade is needed with as many teeth as you can afford.
Because of the rotation of the blade you want the finished surface or your plywood to be on the bottom. I.E. the finshed side of the wood will be underneath and saw will be on top, this will keep the finished side from getting tear out. ( this is the opposite of a table saw )
Go slow and keep the saw fence against the SE. Do some test cuts on scrap wood.