A Slip Stitch Problem!

I am knitting a baby sweater for my cousin, the following link is the pattern:


I have no problem following the pattern until I was ask to do a slip stitch with a crochet hook to close up the first sleeve.

My question is how do you close up the seam, which has two parelle rows of stitiches, when a regular slip stitch with a crochet hook only has one row of stitches???

I am really having a hard time figuring this out!!! If anybody knows how to do that, please response to the message!!! Thank you very much!!!

line up the two rows to be seamed.

insert hook into one side, catch yarn and draw through stitch; insert into other side and catch yarn, draw through stitch and the loop on the hook,.
Insert stitch into next stitch on first side, catch yarn and draw through stitch and loop on hook.

continue to alternate sides.

Thanks for your detail explaination, I finally got it and the baby sweater looks great!!! Thanks a lot:)