Bottom bracket lock ring loose - how should I tighten it
Loosen the lock ring, then tighten the bottom bracket cup until the play goes away and the crank moves smoothly. Then hold the bottom bracket cup in place and tighten the lock ring. 

There are special spanners you can buy for this or you can use an adjustable spanner to move the bottom bracket cup and a screwdriver tapped gently with a hammer to tighten the lock ring. You may have to improvise further with your choice of tools. You may also need an assistant.

A newbie mechanic should have no problems doing this. If you don't try you won't learn.