Here's how it's done in the factory:
You sew the strap or bias tubing, then cut the end of it at an angle. Push the loop turner through the tubing with the latch open. The hook catches the end of the angle, while the latch pokes through the fabric just above it.
Now put the loop over something stationary -- in a factory, it's often a spool pin on one of the 200 lb sewing machines -- and start pulling the fabric back over itself on the turner. Yes, it will scrunch up, but eventually it turns. Start with the bit closest to the hook, and work up towards the handle end from there. Sometimes, with stiff fabric, a little spray of water will soften the fabric enough to turn more easily. When you get it right side out, finger press it and then press with the iron.
Since I don't have a factory machine, my loop turner has a cloth loop on its loop that I can slip over my foot to provide the pull needed to turn the tubing. Watch out you don't hit yourself in the face when you're pulling, too!
Loop turners are really meant for small stuff like spaghetti straps, not tote bag handles.
With wide strips, it's easier to cut the straps 2x the finished width + 2 seam allowances. Draw a line down the exact middle of the length of the strap on the wrong side. Fold under the seam allowance on each long edge. Bring the edges of the folded seam allowances to meet on that line you drew. Sew down the middle of the seam allowances, like you were topstitching but really far from the edge, and also topstitch the edges.
Glue-basting with a little Elmer's school glue can help the sewing effort with something like this.