I use a sheet of black craft foam under the sandwich, smaller slightly than the contact glass itself. Then I place the print paper face-up on the sheet of foam. Then I place the paper negative (RC paper) face down, centered as well as I can on the print paper. There's usually some curl to the paper negative, so it's hard to center the negative properly at first. Next, I place the sheet of glass over the sandwich, then with little or no pressure on the glass I move it laterally, which has the effect of sliding the upper paper negative against the lower print paper, enabling me to fine tune the alignment of both. Then I apply full pressure on the glass and hit the switch for the printing light source.

I think the black craft foam helps the lower print paper to stay put while the upper paper negative is permitted to move.

~Joe