This is the easiest way to clamp a picture frame and hold it square while the glue dries.

You will need-

Braided tape,
8 small blocks,
A picture frame ready to glue up,
Some nails,
Glue,

Step 1: Set Up for a Dry Run.

Lay out the frame face up on a flat bench in the position it will be glued in.
Then place the blocks in the position you see in the picture.

Step 2: Strap It Up.

Wrap the strapping around the frame and blocks and fix a clamp to hold it from slipping, or use a locking tiedown like the one shown. You don't have to have it tight. Just enough to be able to slip it off easily. let's do that and glue up the joins.

Step 3: Glue It Up.

Apply glue to the joins now and set the frame up the same way you did in step 2.
Put the strap around and move the blocks towards the joins. You can see the strap gets tighter as the blocks spread apart.
This is the time to align the frame making sure the joins are flush.
If the 45° cuts are true and the blocks are relatively square, also the parallel sides are the same length, the frame will be square when it clamps up. But it won't hurt to check with a square.

Step 4: Wipe Up the Excess Glue.

You will need to keep the frame on a flat surface while the glue dries. But if there is glue under the join, when it dries it will stick the frame to the bench / work surface. So as you clean up the excess glue don't forget about wipeing underneath.

Step 5: Finish Up.

You could shoot a Brad into the join or drill a hole and send a screw into the join to give added support. The glue join will be as good as the glue you use, so added fixing might be needed. Do it after the glue dries.