I take it by "skill saw" you mean "skil saw" or hand-held circular saw.
Lotsa things can cause such a saw to leave a rough edge, besides the
blade- arbor end-play or out-of-trueness, and your lack of an edge
guide would be my first suspects. The former are pretty simple to
guesstimate; if bad enough, the saw will at best leave a rough cut
face. For the latter, you'll have to devise an edge guide to clamp to
the workpiece to guide the saw for a straight, square cut.

Hand-guided, such a saw is good enough for rough cuts of construction
lumber, in competent hands, and cutting kindling.

If you look and listen here a while, you'll notice that any cuts such
as you seem to want to make would typically be made with a miter saw of
some sort, or a "radical-harm-saw" or a table-saw.

HTH,
John