How to SL1 at the end of a row?

Hi! I’m brand new to knitting! The blanket pattern im following starts with:

Using color A, CO 99 sts.
Set Up Row: P to last st, sl1.

Ending rows with SL1 continues throughout the pattern.

Im soo confused about the slip stitch. If it says purl to the last stitch then how can I slip a stitch? Most videos and tutorials just start with the slip stitch since its transferring a stitch from one needle to another needle. Also I always end up with too many stitches! im sure its an incorrect purl/stitch yarn placement but I think the SL1 at the END of the row is also messing me up

Any advice is greatly appreciated!!!

You work all but the last stitch, then slip it. When a pattern says “work to the last stitch” they mean ëverything but the last one, then do this."
If it’s consistent through the directions, you could just as easily slip the beginning of the next row and do the same thing throughout, assuming it doesn’t affect the stitch pattern. When you slip a stitch at the beginning or end of the row, it makes a neat edge (where you can spare the stretchiness, because it isn’t quite as soft an edge.)

There’s a video on here about extra stitches that might help you. When you’re getting started, it’s easy to get things in the wrong spot now and then. In a month or so, you’ll wonder why knitting seemed hard!