Very pretty! I think that your challenge in finding the stitches (2 legs) and rows (1 bar) for seaming is more likely a result of the garter stitch pattern than the cotton fiber. The big challenge with garter stitch is that its unique pattern of knit row, purl row, knit row, as viewed from one side only, means that you are only really looking at the loops that form the stitches, rather than the legs of the stitch, as stockinette produces on the right side.
My best advice on finding the stitches is to take the top visual row of the sleeve cap and fold it away from you, as for a hemline. What you will see when garter is forced to flatten this way is the legs that correspond to the stitch loops which would normally be all you see of the fabric surface. Those adjacent leg pairs that look like \/ are the ones you want. The pairs that look like /\ are the inside legs of two adjacent stitches.
For finding the rows, you just remember that stitches stack in side-by-side columns. The challenge in this case is that when seaming along a column of garter stitches, the bar between the columns of stitches will alternate between being at the near surface of the fabric and the far surface of the fabric. Stretch the fabric gently sideways to separate the columns of stitches a bit, and you will quickly see what I mean.
Hope that helped. I'm sure you will do much better than I did when starting out! I unwittingly set myself on some truly spectacularly challenging paths when I was too short of knowledge to know how truly short of knowledge I was. Live and learn. I will try to check back on your progress in a day or two, but if you have the need, drop me a note and I'll be happy to get more specific.