Please help! Mattress stitch question

Hi, I just learned the mattress stitch to sew seams in stockinette. The trouble in when I am trying to sew together two edges of a sleeve (not attaching it to the sweater yet, just closing the sleeve itself). Because an increase was used along the outer edges there is no straight line of stitches up the edge to sew together. What do I do to keep it even?
THANKS!!!

You have to move your mattress stitch over where the increases are. Don’t worry–it’ll look great. Love the mattress stitch!

So work up the line of stitches until you come to where you increased, then move closer to the edge by a stitch.