If you want to see on Youtube the process I am going to describe in more detail, see: How to Sharpen End Mills on a Surface Grinder Part 1 with Dale Derry.

After sharpening his two flute end mill (as I shall describe how to do it his way also and with a variation on it), he puts the previously dull end mill to use and then breaks it showing the limitation of what end sharpening will do for you in that if you only sharpen the end teeth on an end mill and not the spiral flute teeth (if they are dull also), one is limited to making shallow surfacing cuts (which is OK by me as it just involves using up more of one's time but since I am retired...) and as well there is an increased danger of chipping a corner if you really go at it but with the speeds and feeds I employ not likely. I look for a good surface finish and can I produce square and parallel surfaces...

The sharpening of the spiral teeth on an end mill is a whole other process and there are limitations here also besides reducing the diameter of the end mill (I love undersize end mills and use them all the time), but with repeated sharpening one changes (reduces) the cutting teeth positive rake angle and if it is in the form of a hook profile also and most HSS end mills have this feature (as the hook promotes free cutting and a fine surface finish while a positive tooth rake angle reduces end mill deflection and has a lower power requirement vs a neutral or negative rake cutting edge). And when the positive tooth rake angle is compromised, the performance of the end mill is seriously degraded but I digress... So there are limitations whatever you do.

I will make up a sketch to illustrate the above but I am no great artist.