Never done it before. Received a bunch of .375 straight wall brass and want to neck it for .375 Ruger. Is it as simple as running through .375 Ruger FL die or .375 Ruger neck die?

Pretty much; try one and see if it needs annealing first. Do not try to horse it all in at once; milk it in and out (Hard to describe), and redistribute the lube each time; do not use too much lube or you will dent the cases, and don't use too little either.
The necks must be PERFECT; otherwise you will get folds in them. I mean the mouths.
Saturday I did some 577-450s from 24 gauge brass; that is the ultimate in forming. Except for the .22-.50 BMG.

Also I might add to dpcds post that it will help if you neck it down in as many steps as possible such as to through a 416 die then a .375...Milking as described is some good advise for sure.

Another procedure is to use file trim dies for a 416 Rem, 416 Ruger and 375 Ruger then trim and full length resize last of all. the Idea to bring it down in as many gradual steps as possible.