i can just edit makepkg.conf and uncomment the line so it reads like this:
BUILDDIR=/tmp/makepkg
??
anything else i need to do?
So long as /tmp is already mounted tmpfs that will work.
Bear in mind certain packages need a lot of space to build. Depending how much memory is devoted to /tmp you may have to switch back to using the ssd for such packages or temporarily increase the memory allocated to /tmp, if you have enough available.