I've been trying to make 350 Rem Mag brass out of 7mm Rem ,300 Win and 338 Win. What I've read to do is to take a magnum case and run it through a 350 die with the decapper/expander removed and then trim to length. What happens on all the brass regardless of caliber is that it creases at the shoulder neck juction. What am I doing wrong. Any ideas?

Two things...350 Mag brass is still available and you're going to a lot of trouble.

If you want to use other brass you are going to need to anneal it first...I assume you are using old brass but even if you aren't, annealing really helps. So after you anneal and you expand the necks and then you run them thru your .350 Mag dies you can trim them to length.
Then you need to inside neck ream your cases as the brass in the body (where your neck is now) is thicker than in the neck. Lot of thouble isn't it?

Are the creases from the brass buckling or dents from excessive lubricant? It's very easy to get too much lubricant in the die in an operation of this type. If they are lube dents, then just load them and shoot them out.

On the other hand, if the creases are from brass buckling do as DB says and anneal a used case, or better yet, if you start with new cases they will likely not need annealing.