I recently opened the modified choke on a 410 Wingmaster by about .003" with a reamer from Brownells. The reamer is about $45 and it did a fine job, requiring little polishing.
I think that almost anyone who is even a little bit mechanically inclined can do this job, but it is smart to approach it slowly.
First of all, buy the extension and work from the chamber end rather than the muzzle. That way the straight portion of the choke will be closer to being in line with the barrel when you are done.
Secondly open the choke in small increments, say .002" at a time and shoot 3 or 4 patterns each time to see how it is comming. When you get the pattern spread you want, quit. As a matter of fact, it won't hurt to quit BEFORE you get the spread you want and shoot the gun for a while----you can always take out another couple of thousandths, but you can't conviently put them back.
You should be equipted with a 1" micrometer and a telescope gauge to measure the muzzle diameter and to set the reamer, as the reamer is, of course, adjustable.
