When I had my son (who is now 5 1/2), I got this little jewel as a gift.
Now I attach the Velcro. You will have a male and female piece of Velcro; one will go on one end of your strip on the top and the other will go on the other end on the bottom.
Yup, that's what I'm using! I took the mesh bag (seen in the corner of the above picture) that came with the laundry hamper and measured what I needed. This particular bag had two side pieces and one long gusset. So I cut out my fabric from the bolt.
My original measurements for the bag were as follows: the side pieces were 16" x 19". The gusset needed to be at least 54" long by 15.5" wide. My fabric was 55" wide so I just left it. Basically I had one really long rectangle and two short ones.
The first step is to line up the fabric wrong side to wrong side. I know that doesn't sound right but go with me. You will sew along the area that needs to be sewn with about a 1/4" seam.