Use a 15mm combination spanner.
Put the belt on the top pulleys (Alterrnator etc)
Thead the spanner under the belt (this is so the belt ends up the correct side of the spanner when you lever over the tensioner)
lever the tensioner over using the open end of the spanner (anti-clockwise) , then wedge a long screwdriver through the ring end so that it sticks between the brake caliper and the cars body. (use the handled end on the brake caliper).
You can thread the belt around. Then release the spanner screwdriver.
Thats roughtly how I do it and it normally works.
Giulietta 2.0JTDm (170), BMW 120d M sport