Steve,
I'm a newbie, but your request has been up a long time without answer. So I'll relate my experience, which consists of having done 1 practice project with therm feet and other Federal details. I was trying to learn stringing, inlay, therm feet, cockbeading. If it is practical for you, sign up for a course with Rob Millard or Garrett Hack, or someone else who brings background and experience to the project. With that caveat, here goes....There is a link below to my practice project. I did it all with handtools, but you might make good use of a band saw, table saw, and/or router.
When I got started, it seemed to me that shaping the profile would be the most critical part of the therm foot. But as I got into it, I realized that the actual shaping is not what is most difficult. What was most critical was the definition of the arrises and reference lines which terminate the curves--if you get those right, the actual shaping will follow naturally.
I strongly suggest making 5 (at least) leg blanks, so you can use one as pure practice, and can discover all the keys and pitfalls for yourself. As soon as you have your leg blanks finished, you begin the definition of the foot by sawing the upper limit of the shoulders--this is where your taper will terminate. Sawing these shoulders is scary, because all errors are doubled. So these are precision cuts. I used a hand saw, but you might want to use the miter gauge of a table saw. For example, if the leg is 1-5/8" square, and you want the leg to taper to 7/8" square right above the therm foot, then you want to make a 3/8" depth cut girdling the four leg faces, to define the top of the foot--you're going to taper to this point. Suggest making that cut a hair less than 3/8" to allow for final detailing of the leg after you complete the tapering. You can take a little more off, but you can't put a little more back on.
Now saw the tapers in whatever way that suits you, and clean them up. At this point, you have to decide whether you want to string and inlay the legs next (I'm looking at p. 340, Mussey), or whether to make the therm foot next. To make the foot, the next thing I suggest is to use a handsaw, table saw, or router to cut the step between the upper bead and main body of the foot. Cutting that step will define the lines terminating the upper curved surfaces. If, instead, you immediately start carving or rasping, the whole thing will soon be swimming in front of your eyes, because you're trying to cut your reference lines as you go. Once you cut the step, then the upper lines of the foot are defined, and it's a matter of rasping and filing and 320g paper to form the upper surfaces. I found that an Auriou 14-grain rasp (it's quite fine) was very useful for this part, followed by 2-cut and 4-cut files (European grading, e.g., Grobet). I think you will be able to form the curves by eye, but as an aid you could make a template right off your drawings, and use the template to check your progress as you go. By the way, if you don't want to invest in expensive rasps, you can do the same thing with PSA sandpaper on the right sized sticks. Also, I suggest masking off the bottom of the legs right above the therm foot, to save them from stray tool marks or sandpaper marks.
I found the lower surfaces as tricky as the upper ones, especially the increasing curve into the bulb of the foot. Lay the curves out, and I suggest using a coping saw or coarse rasp to rough away most of the waste. When you start getting in range, shift to spokeshaves--what the shaves do for you is maintain smooth curves and neat edges for the lower foot, because these edges need to curve up smoothly and meet the edges of the bulb very precisely. The shaves give you enough control to do this. The shaves I found most useful were the LN small bronze shaves. The curved one has about the correct radius for the fastest part of the curve, and the straight shave is useful for the lower foot. The shave may leave a slightly rough surface right under the bulb, because you're shaving end grain in there. Detail these lower surfaces with 14-grain rasp and files and 320g as before.
Well, as I said, I'm a newbie myself, but hopefully some of the above will help you focus your own thinking. Take a class if it is at all practical, and get the benefit of expert advise. That way, you will also get the best advice on the precise sizing of the legs and details of the foot--this is very difficult to scale from pictures.
Wiley
P. S. Having a tough time posting a photo here. My practice project can be seen here:
http://www.traditionaltools.us/cms/index.php?name=coppermine&file=thumbnails&album=35