I always use an angle grinder and it can be difficult to keep the proper angle. You have to do it enough to get a "feel" for it. I tend to need a lot of hand file dressing to clean it up. I have sine acquired a bench grinder I think will be much easier. Find the right angle and then slide the blade to the side. The dremel attachment I haven't used so I don't know how well it executes but it should be what you are looking for. It will just be a guide to keep you from wavering side to side. Even if it's not perfect it should be better than freehand. If you don't like the attachment and were just doing it with a dremel by hand, you might want to try with just a hand file. The longer the piece the more consistent the path. Less dips and lumps as a 12" file will glide across them instead of falling into them like a small rotary bit.

I don't think there is a secret to hand sharpening a rotary lawnmower blade with a file, but the task gets a whole lot easier if you can figure out a way to support the blade and hold it in position at a convenient angle. A sandbag can be used, or some wooden blocks of the correct size. Once you achieve a stable set-up, it's pretty easy to file the blade down. It does not require super skill, or super coordination.

I can't even imagine using a file or a Dremel to sharpen blades in a commercial setting. They are usually so damaged that it would take an hour of hard labor to sharpen.

Not everybody has an angle grinder and 1" belt sander or the skill to use it, but that's the way I can do it.

I sharpen my blades with a file.

First, I clamp the blade to the corner of my work bench with a quick release bar clamp. I then hold the file with both hands. One hand is on the handle, the other on the opposite end. I then take the file and follow the existing angle and draw the file away from me, using moderate pressure. I think the last blade I did took 50 draws on each side of the blade. Finally, I check balance.

Total process takes 5 minutes, and the blade looks like new.