Hiya - T.H.A.P.F is one of the first pieces I'm currently finishing off. It was probably the piece that inspired me to take up piano in the first place (although i played keyboard before).
THe way I 'learned' the piece: (i used inverted commas because I dont technically play it 100% accurate, but enough to bring out the tune well)
1. Right hand only, first page. Learn it chunk by chunk (e.g. intro, then the first part of melody, then the melody played a few notes higher etc)
2. Right hand + single fingered chord on 1st page
3. Repeat process on second page, one hand only, then single left finger. (again, work on each segment of the tune seperate, and put it together)
4. Work out the intro with two hands together
5. Work on the first page - in particular, get the pattern going in the first few bars of the main theme with the left hand - it is repeated all over the first page, only starting on different notes. It you can nail it in one position, it should be fairly easy to move the hand to get the others.
6. THis is where I fall down. Another pattern is used for the left hand on page two, but I cheated and used almost the same pattern from page 1. In my desperation to hear myself play it, I rushed and ruined it. It stills sounds ok tho.
Thats about all I can say. I would say, from my own experience, that it is a very mechanical piece of music, with a beautiful melody. And its really fun to play (even incorrectly). Just slow things down, bring them up to speed, and have fun!
p.s. it took a while for me to get from step 3 to step 4. Incorporating the left hand fully seemed like such a big gap, but I didnt find it too difficult after a few days. I started the piece in October, and i'm just finishing it now. (not that that means anything, different people learn different rates, and as I say, I kinda cheated
And my advice is only from my personal experiences and i dont consider them to be the proper way to do things. Only my opinion.