Four weeks ago we went from Langkawi to Krabi in one day. It was very easy, no problem at all.
In Langkawi we went to the jetty in Kuah. There is (between a lot of counters), one counter of Langkawi Ferry Services which runs four times a day a boat to Satun in Thailand. Here you can buy tickets. The tickets are very cheap, about 5 euros p.p, can't remember the price in Malaysian Ringgit, but the equivalent in Thai Bath was 250 Bath p.p. You get a boarding pass with the number of your seat. The seats are like in a small airplane with two rows, each side with three seats.
We were happy we were more then one hour before the boat left at the ticketcounter to buy the tickets because i think the boat was fully booked when we left. After buying the tickets you head for a waitingroom and then pass the customs to get on board of the ferry. We took the first boat which left at 9.30 am (if i remember right). Maybe it was 10.00 am, i am not sure. The times of their website were not right so you better give them a ring: (www.langkawi-ferry.com).
It takes about one hour to arrive in Satun at the Tammalang Pier. Because of the timedifference you arrive at the same time as you leave in Langkawi. At the harbour of Satun we had our rentalcar still waiting (we had hired a car in Thailand and drove from Bangkok to Satun, then took the ferry to Langkawi and returned 6 days later).
By car it takes about 5-6 hours to drive to Krabi, Ao Nang. We arrived there around 3 pm, so no overnight stopping necessary if you take the first boat in Langkawi.
I guess you will not have a waiting rentalcar. In that case you can easily find transportation at the Tammalang Pier of Satun. In the building you arrive and pass the customs there is an official counter were you can arrange transportation, i think i remember for fixed prices. I didn't look good enough for we didn't need it. I think there was a bus-service as well. The Tammalang Pier is in the middle of nowhere, kilometers away from the town of Satun, so you really have to make arrangements at the Pier yourself, for you can't walk to the town and there is nothing around.
The whole trip was very easy and entertaining, the boat was good and i even didn't get seasick at all.
If you have any questions left, i will be happy to answer.