It's fairly easy to swap feet, but a second pair of hands helps a lot, especially if your doll is strung tightly (Ashley is not). Here's the elastic going up into the ankle and foot:
Then take your flat foot and test-fit the ankle ball into it--if it isn't going to fit, you might have the wrong foot, and the time to find out is not when you are wrestling with the elastic in mid-air.
Now put the ribbon through the ankle ball. Be sure that the extra piece is pointed away from the leg.
This is the hardest part. If you have a second person, have them hold the ribbon ends and pull the elastic up through the ankle piece. If the string is tight, you might have to yank out the crochet hook at this point to get a little more slack in the elastic. The smooth surface of the hook makes it easier to do this in a hurry if you have to. The idea is to pull enough elastic up you can hook it with the foot hook. Of course the hook is sort of inside the foot, so it's a challenge. It might be tempting to hold the ends of the ribbon in your teeth, but don't do that--if it's really hard to get enough slack on the elastic, you can use your thighs as a vise for the whole doll, and that frees up both hands to manipulate that elastic. You might want to remove the wig or faceplate if your doll is a faceplate doll like Gneiss.
Anyway, the foot is on!