I recently sank a lugged Wetterlings head on an overbuilt shoulder, and hopefully I can explain this well enough in words to give you some idea, because I have to say on my first try it went rather smoothly.
First, I focused on shaping the haft with 100 grit sandpaper and a Mora so that it very securely fit into the eye of the head. I pounded it down about a quarter of an inch, popped the head back off, observed pressure points where paint from inside the eye had worn off on the handle, and proceeded to sand the handle down the kerf to size. Then, I put the head back on, pounded it down about an inch, removed, reshaped, repeat. When it came time to shape the area for the lugs, I drove the head on, backed it off about an inch, and began carving out pressure areas where the lugs were going to sit. This was actually really easy to do. I carved from the shoulder up towards the eye, so that I wouldn't split the haft farther than necessary, and once I had the necessary material removed for the lugs, sanded it all out really good so that I didn't give the shoulder any opportunity to split around the carved out area. With a sharp carving knife, you can make quick work of that material, but always err on the side of caution. If you feel like you're going a little fast, take a step back and slow down-you don't want to wreck a haft that you have an hour of time invested in.
If the shoulder area is overly bulky from the get go, I'd start with removing a good amount of material there-supposedly an overbuilt shoulder increases the chances of splits generating down the handle.
