First, make sure you got a stone for sharpening HSS - Borazon is best or soft friable grade pink stone. Or if you're doing Carbide about the only thing I've had luck with is a diamond wheel - the green one are supposed to work though.
For your first few, use a larger cheap HSS endmill that doesn't need a lot removed. The bottom of your fixture has two flats.One flat gives maybe 20 degree relief for the secondary relief. The other flat gives maybe 4 degrees relief for the primary relief right at the cutting edge. The unit has little ball detents so you can easily rotate the endmill exactly 1/2, 1/3, 1/4, 1/6 of a turn.
This is hard to explain, easy to show. Tighten the endmill and collet in the holder so the cutting edge goes straight across. Then place the unit in the surface grinder so it grinds on just one flute. 1/2 the endmill is under the stone and one flute is coming up right along side the stone. This has to be CLOSE but not touch. Too far way and you'll end up with a tit in the center of your endmill after you're done. Too close, you'll sharpen the leading edge off the next flute with the side of the wheel.
Once you're set up slowly lower the wheel and grind off a bit on one flute, back and forth. Then rotate the endmill to the next tooth and grind it. (there are twelve detents on mine, so 6 clicks is 1/2 turn, 3 clicks is 1/4 turn etc etc) Go all the way around, lower the wheel and repeat. On a very slightly dulled EM you can do only a few licks on the primary relief. On a bad one, I do the secondary relief first, then finish with primary.There's specs on the amount of primary relief, but just look at it - the land on the endmill should be the same as a new one. On a REALLY bad one, get the endmill close by hand in a bench grinder first and save some time.
This will give you an endmill that's not center cutting, and has no "fishtail". But works real good for most applications. For center cutting, you'll have to learn to gash the endmill first. I do this by hand with a die grinder and cutoff wheel under a 10X magnify light. For "fishtail" (center of endmill relief from edge), you can set the fixture on the surface grinder at a 30 degree angle, but with the endmill itself square to the wheel. (Again - easy to show; hard to explain)
This will take some playing to learn. Spend a day or two screwing up. After you get on to it, it only take a few minutes to sharpen the ends.