Set the NSPR version number. Five files need to be updated: configure.in, configure, repackage.sh, prinit.h, and vercheck.c. Look at their CVS histories for examples.
Make a dummy change (add or remove a blank line) to mozilla/nsprpub/config/prdepend.h.
Make sure the NSS tinderboxes (which also build and test NSPR) are all green. The build+test cycles of the NSS tinderboxes are very long, so you usually need to wait half a day for them to cycle through.
Create a BETA CVS tag. The naming convention is NSPR_x_y_z_BETAn for NSPR x.y.z Beta n.