Most of the bags I've made have had a strap on each side, but I've done a few that only had one strap.
Either way, you want to be sure to top stitch your strap(s) close to each edge before you attach so it will lay flat even after washing. If the fabric is not very heavy you may want to use interfacing in the strap for stability.
When attaching the strap I usually leave more than 1/2" of the strap inside the seam. I first stitch the strap to the outside fabric just inside the seam line. Then when I stitch the lining to the outer fabric I use a slightly larger seam and will often back stitch over each strap for additional security in the seam.
The main thing when attaching your strap(s) is to make sure the strap is not twisted.
Hope this helps! If you have further questions PM me and I will gladly try to answer them.