The original factory edge on your MDF, ply or whatever will be pretty straight and smooth so your first cut will be reasonably ok. Your saw will almost certainly leave a rougher edge than the original factory one and that is your reference edge for the next cut. Most of the edge guides that come with hand held circular saws, especially the cheaper ones, are fairly flimsy and short and can flex with slight changes in pressure etc as you work your way along an 8 ft cut. This leads to a less than perfectly straight edge and with successive strips these little deviations start to accumulate. With a sawboard the sole plate of the saw is always registered off a factory edge so even if you allow the saw to wander off the fence you can correct it with a trimming cut and start again. As with most things a little practice helps a lot. Supporting your board while you work is another thing to consider at this stage. I use a grid or cutting table made from three 8ft 4 x 2s and and two 4ft 6 x 2s supported on saw horses. With this cutting table the work is at a convenient height and fully supported before, during and after the cut. Other people use 8' x 4' sheets of insulating foam board laid on the floor to support the workpiece. Just remember to set the saw to a depth of cut just a little more than the thickness of the sheet to be cut (that is good practice whatever method you use).
