make a copy of your cvs repository (not a checkout, but a copy)
cp cvsroot tmpdir
checkout the copy, cd into the checkout
TZ=UTC cvsps -u -A -q > ../cvsps.dump cd .. TZ=UTC hg-cvs-import cvsroot cvsmodule hg-rep mapfile < cvsps.dump
module is the cvs module (ie cvs checkout module)
mapfile should be empty the first time the import is run. It records a mapping of hg csets to cvsps commits.
As your cvs repo updates, you can run cvsps again and then run the importer again to update. As long as you use the same mapfile, it will pick up where it left off last time.