One of the challenges when building a chair is clamping the dang thing down so you can work on it. I’ve seen lots of solutions that use band clamps. But I dislike band clamps (perhaps I had a bad experience at band camp). So here’s what I do.
Most workholding problems can be solved with handscrew clamps and holdfasts, including this one.
First you squeeze the legs with the handscrew clamps. Note that if your legs have any rake or splay there are going to be only two angles on each leg where the handscrew will be able to get a good grip. If you’re a chairmaker, you know that this angle is called the sightline.
The sightline is the place where the leg appears to be vertical to your eye. To find the sightline with your handscrew, rotate its jaws around the leg until you find the spot where the clamp will grab the leg without the clamp body’s twisting.
(Note: I have some grippy stuff on the jaws of my handscrews – it’s adhesive stair-tread tape and it prevents things from slipping.)