The Welsh Wildlife website will give you all the information you need to find out how to plan your trip to Skomer. You have to catch the little ferry from Martins Haven in the morning as early as possible to sail across the short distance to the island. If you visit the Trip Adviser page and have a good read you will get some tips on what to take and when to arrive in order to be sure to catch the small boat. Visitor numbers are limited and you cannot book much in advance because of the weather so you do have to go prepared to wait your turn for the boat. I think this is all part of the adventure!
The Skomer puffins arrive back from their winter habitat in the southern hemisphere towards the end of March and normally the first chicks hatch out in May. Ideal weeks to visit are in June after the Whitsun week which can get very busy. The small birds have normally flown away by the end of July.