This is especially true of flat grinding.
If you don't want to make it, or want to see how to use it, just go here, this guy invented the idea,
Basically it's a magnet a bubble level and a 1 dimensional gimbal to allow you to adjust the angle.
Push a knife whos grind you wan to copy, say a Mora, to the sander, click the jig on to the blade,adjust till the buble is in the center of the level... you can now copy that angle exactly while grinding.
The Bubble jig guy sells wedges too, that are at the common knife grinding angles.
Also you can just use a adjustable protractor or angle finder.
but lets go through the steps on how to make your own for a couple bucks.
1" or 1.5" aluminum angle 3" in length or more.
1 1/4in wing nut
2 1/4 in lock washers (see pics for the kind i used)
1 14in by 1.25in pan head machine screw
1 .75-1" diameter rare earth magnet
1 cheap bubble level, i could only find the ones with 2 levels(like this), so i just cut it apart.
You can see a completed bubble jig in the bottom let corner of this pic, I'll try to get a better one up later.
I made it at Techshop!