Make a chain a little wider than the width of your phone. then do a sc in each ch across. Make 3 scs in the end, depending on how wide you want to make it, you can add more, My phone measures 6 x 3 inches in this photo. Then sc in each ch down the other side, do 2 scs in last ch. Do not join-work in continuous spiral throughout.
After about 2-3 rows, check to see how snug your phone fits. If it's a little wide, just make a couple of sc decreases, 1 on each side should do it, do more if you need ti. Then just continue until it's the height you want. Add your strap by doing a chain as long as you like, go back over with a row of scs. Connect at the opposite end. The one i made here also fits a water bottle. The final measurement came out to by 7 x 4 - one inch bigger than my phone.
I wanted mine to be a bit thicker as to add protection, so i used one strand of ww acrylic and one strand of wavy homespun yarn held together.
I do have a few of these in stock if you would like to purchase them:
Cross Body Cell Phone / Water Bottle Pouch iPhone Sling Carrier Light Blue