Travel from Krabi to Ko Pha Ngan

well the only way off Phi Phi is by ferry or speedboat, so getting to Krabi will be part of any method. Most methods will also require you to go from Koh Samui and from there to Ko Pha Ngan by boat/ferry.

From Krabi you can either go by air - Bangkok airways flies direct to Samui, or overland. The overland route will take you via Surat Thani and then to the ferry to Koh Samui. You can go overland by either regular bus service, or through a tour company. (iirc we paid about 600-700 BHT for the overland method from Krabi to Samui including the ferry tickets)

Easiest way is by combined ferry/bus/ferry ticket, any travel agent can sort you out one of these combo tickets.

No need to stop off, the journey can be done in a day.

Just a heads up, if you buy an all-in-one ticket from a travel agent, you may well end up travelling on a bus operated by PP Family. There have been bad reports about this company being involved in the theft of items from travellers luggage, so keep your money and valuables on your person and don't leave anything of value in your main bags in the hold of the bus. But it shouldn't be too much of an issue if you are travelling during the day. I have used PP Family a number of times and haven't personally had any problems.

You don't need to go to Ko Samui first as said above. Plenty of ferries stright from Don Sak pier (mainland) to Ko Phangan. As Dominic says, just go to a travel agent in Krabi or Phi Phi and buy an all-in-one ferry-bus-ferry ticket and keep any valuables with you. If starting early you should be on a Ko Phangan beach by the afternoon.

good info about bypassing Samui - I did not know there was a direct ferry (we stopped in Samui for a few days anyway so did not look for direct routes), would save you a few hours and the extra $$ for getting from Samui to Pha Ngan