I Wouldn't screw with the mitt, as that is not the way you normally seam carpet. I would just get a garage sale iron and know it will have that glue residue on when you are finished. Cut the desired length of tape, heat with iron on one side of tape and attach to main body. Then on other side heat with iron as well and carefully place the cut piece on top of tape and butt up as closely to main carpet as possible without overlapping.. This is where the roller comes in so it presses the carpet to the tape. If needed use the glue gun on areas that didn't stick as well and use the roller again. Then trim to finish off cosmetically.
Have Fun!