Novice Knitting a Child's Vest, armhole problem

I’m new to knitting and new to this forum. I purchased an “easy” vest pattern on Etsy to knit for my two grandsons. The picture looked like it would be simple enough and the shop owner described the pattern as “Easy.” I am, however, having a problem where the armhole begins. The body of the vest is Stockinette and the area around the armhole is Garter. 8" into the body of the vest I am to start using the Garter stitch for 10 stitches at the beginning and end of the row and the Stockinette stitch for the area in between, just like the rest of the body. For some reason, the two sides of the row don’t look alike, which I’m sure they should. I don’t know what I am doing wrong. Does anyone have a suggestion as to what I am doing wrong? Also, I don’t know if it matters if I begin the armhole stitches on a knit row or a purl row. I would appreciate any help offered! Thanks, LillyPatch

Welcome to the forum!
You can do garter stitch by purling sts every row or by knitting sts every row. Mostly patterns use knit sts.
Did you start and end a purl row with 10 knit sts to begin this part of the pattern? Then you would knit all sts across the knit row. Repeat these two rows.

Thanks for the reply. I went back a couple of rows and began again. This time, somehow, I did it right. I really appreciate your advice. This forum is terrific.