Firstly I should mention the blades I'm sharpening here are massive old things, 1/2" thick ! The same method works however with any planer blade although thinner blades will heat up faster and will that would need watching. A bucket of water beside the grinder is a good idea especially with thinner blades.
I use a cheap grinder with the grinding stone bevelled to about 30 degrees. The photo will show the slide along which I move the blade and the bolt tapped through the timber that is the blade height adjustment.The screw is in an oversized hole and allows the steel slide to rise as needed while holding it to the frame. I move the blade back and forward at about 2 inches a second being very aware of heat buildup .
This shows the resulting grind, it's extremely even with a 3" radius. I always put a small micro bevel on the edge with an oil stone, it strengthens the edge and lets it last longer. It's very easy with the hollow grind as you only have to remove a tiny amount of steel and the edge guides the oil stone.
Another minor advantage of sharpening your own planer blades is the you can decide whether to completely remove nicks and edge damage. Sometimes you can just offset the blade a small amount so the nick no longer effects the finish. Your sharpening shop will remove all nicks ....which is good as long as you don't mind loosing an 1/8th of an inch of blade each time.