This youtube link is one way to trim brass.
However, because I work with my hands so much, I tend to get major hand cramps from working with brass because it is so tiny. So I went a step further and this is what I came up with.
In the first pic I have A) an RCBS priming tool base. I drilled a .5" hole directly under the shell holder in the base of the priming tool. Then as you can see there are two more holes drilled on opposite sides of the base. These holes help sucure the base to my drill press. Do not drill these holes until you have the priming tool on the press and lined up with the drill chuck. Then drill the holes to match the fence grooves of the drill press base and that way it will fit the press.
B) This is a simple .5" diameter hex head fine thread bolt. The purpose of this bolt is to align part C (a 1/4" drill bit) flush with the inside of the shell holder so when the Lee Trimmer comes down into the primer hole of the brass, it stops flush with the portion that the brass is sitting on. This is very important otherwise your brass trimming will not be accurate because the pilot on the end of the trimmer will go past it's stopping point. Also the drill bit needs to be flush so the brass will smoothly go in and out of the shell holder. As you will notice, there are two nuts and two o-rings between those two nuts. That way you can tighten them without applying alot of presure and hold accuracy. It makes it easier to manupulate the height of the drill bit by having the o-rings in between the nut and the press plate. Use a bolt with an allen head, it makes it easier to adjust and I prefer fine thread for presice adjustments.
C) Once again, a 1/4" drill bit. I used a drill bit because it is hardened steel. Make sure that both ends are flat and smooth. Use a grinding wheel to get it that way.
D) A simple Lee Trimming tool. I had it necked down by a machine shop to fit the chuck on my drill press. It was cheaper than a chuck. The trade was, I trimmed a bunch of brass for him.
The next pic shows the Priming tool sitting on the press with the .5" bolt installed into the base plate. Keep in mind, there will be the bolt head underneath the plate of the priming tool, with a nut, an o-ring, the base plate, the next o-ring, and then the 2nd nut. Adjust the .5" bolt so the drill bit is flush with the shell holder base. Now put in the shell holder you need, and place the brass inside the shell holder. Next install the Lee trimmimg tool into the chuck with the proper cutting tool installed. Next raise the drill press platform up guiding the trimmer into the brass so it is almost all the way in. This is when you tighten the priming tool to the press. Now you lower the priming tool, (well call it a jig from here out) and only lower it so you can comfortably take your brass in and out of the shell holder. You do not want anymore travel than is needed for your press other wise it is just more work when there is too much travel. The idea is minimal travel for the cutting tool. Now lower the trimming tool (drill is not on yet) into the brass and make sure it is perfectly centered and true to the brass. Now tighten the drill press plate. Lower the trimming tool into the brass several times gently. The trimming pilot should fit smoothly into the primer hole of the brass. The jig should still be firm and solid.
Set the drill press to the lowest speed. I prefer to use a leather glove on the hand that is placing the brass into the shell holder. Keep in mind that the brass will want to spin, but all you have to do is put a little pressure with the leahter gloves against the brass and the brass will stay put for the brief moment it is being cut. Once trimmed, there will be minimal pressure on the brass. One very important thing is make certin not to push to hard with the press. I put very little pressure into it when I am coming down with the press because it doesn't take much. Too much pressure can begin bending the trimming tool. I have never bent or broke one yet so just apply enough pressure to complete the cutting and that is all.
Accuracy is the key to trimming. When I do this for other people I always let them go through and randomly measure their brass before they pay. I have never had any problems and people appreciate my service. With this setup that I have now, I can trim for hours with minimal effort.