A lot of 'elbow grease' .. seriously.
I basically started with coarse grit paper to 'rip' the majority of the powder coat off, being careful enough not to cut through to the aluminium, as that would mean some deep scratches to remove with the grit I used. Then removed the remaining powder coat and any scratches in the aluminium with progressively finer papers. Always sanding in the opposite direction to the previous grade eg. with the grain and then across the grain. I went down to 1200 grit wet/dry as the final paper. Finally polished by hand with the Meguiars.
The bike was a parts bin special, so sanding and final polishing took place as and when I had a free 5 minutes over many months so I can't suggest how long it actually took.