Hi,
At the risk of sounding trite, practice. A lot of it. What you are trying to do is eye-ball angles to perfection without any guide. Not an easy task.
I've been a machinist for almost 20 years now, and I've ground everything from little pocket knives, to lathe tools, to modifying end mills, to drill bits. First thing to get out of your head is the idea of perfect. Ain't going to happen free-hand. Good enough hopefully, perhaps really close after some practice. But it won't ever be perfect. That takes a jig or sharpening tool. So relax and don't be so retentive.
First thing you need, is good light. If your lighting isn't good enough, you won't be able to see what you are doing properly. I don't like florescent or those curly cue bulbs. Incandescent bulbs are fine, but real sunlight is still the best, IMO.
Let's say the angle you are looking for is 28 degrees. But, we know that we aren't going to be really able to just eye-ball it. But we can probably "see" what 45 degrees is. We can also probably "see" half of 45 degrees pretty good. So if you hold your knife at about half of 45 degrees or just a little bit better, you should be at least close enough for government work

. Do a couple of strokes. Now take a good look at the edge. (This is where the good light comes in). Is the fresh shinny part on the front or back side of your edge? If it's on the backside, raise your angle up a bit, if you can see the shiney on the front, drop your angle a bit. After some practice, you will get a feel and eye for what you are doing. But there is no short cut for learning how, but to do it until you do it right.
And in my book, there is no shame in "cheating" by using a tool to sharpen. In fact I'm all for it. The very truest edges are machine made.
I hope this helps!
dalee