Fold leather around cell phone (leave phone in its case, so pouch will be able to accommodate that, too). Cut leather to about an inch larger than phone in each direction.
Slip out phone, leaving leather folded on one short side. Using leather glue, adhere leather on two long sides, leaving short top side open. Set pouch between two heavy books to dry completely.
Trim excess leather at sides and bottom of pouch with X-acto knife. Take a piece of the excess width, fold it over, and glue it to itself to make a sleeve large enough to hold cord. Glue sleeve to top of back of pouch; let dry completely.
Feed cord through sleeve, then secure ends of cord with glue (or tape); let dry completely. Tuck ends into sleeve.