Hold the LP label and clamp stationary by pressing them against the platter with the thumb and finger of one hand whilst tightening the clamp with the other. No rings.
How tight depends on why you're clamping...
To reduce intra-vinyl resonances:
Try knuckle raps in the deadwax. With the LP unclamped, rap at several points around the dead wax - note the (considerable) amount of ringing and resonance. Clamp a bit and re-rap, noting the difference (reduction) in ringing/resonance. Keep clamping tighter until that's minimized (usually requires very tight clamping).
To flatten a warp:
You can figure that one out. :-)
How tight depends on why you're clamping...
To reduce intra-vinyl resonances:
Try knuckle raps in the deadwax. With the LP unclamped, rap at several points around the dead wax - note the (considerable) amount of ringing and resonance. Clamp a bit and re-rap, noting the difference (reduction) in ringing/resonance. Keep clamping tighter until that's minimized (usually requires very tight clamping).
To flatten a warp:
You can figure that one out. :-)