Here are my thoughts, taking into account that I think you indicated a lathe is not an option. I would make a jig to hold the legs. Rip two strips at a 45, and glue them to a board, so you end up with sort of a trough, with a 90 degree "V" in the bottom of the trough. Orient everything so you can put the square leg blank in the "V" with a corner sticking up. Put a stop one end. Glue a piece of scrap to the bottom so you can clamp it in your vise, or figure out some other way to hold it on the bench. Start by squaring your stock as you normally would. Then put it in the jig, and plane the corner that is sticking up. When half of the original "flats" are gone, rotate 90 degrees and plane the next corner, and so on.
As with so many things, the devil is in the details, and getting those eight sides pretty close to the same size so you come out with a respectable octagon, and not something else is going to take some attention to detail. I have done this with smaller pieces, and found counting "strokes" worked to get things the same size. With larger stuff, I think I would mark the centerline on all four faces before planing. Either way it is going to be a little bit tedious.
In terms of the plane setup, I would use whatever I have that would give me a moderately decent cut without making a mess. In my shop that would probably be my No. 7, and I would look to finish things with a couple of very light strokes from my smoother. I would not get too aggressive on the cuts, even if the first one seems to be going well, because the grain is going to change direction on you as you rotate.
I don't know if this is really what you are looking for, but hopefully it helps, and good luck.