There is an event called Darkmoon Faire that appears in the game once a month, starting at 00:01 on the Sunday before the 1st Monday of each month. During the event you can complete quests for each professions.
The profession quests can only be completed once during each Darkmoon Faire and will grant 5 skill points (means you can complete them again once at the next faire).
The skillups will apply automatically to the most recent expansion profession progress that is not at its maximum. So, for example, if you want the +5 skill in Outland Engineering, but you have already learned other expansions profession skills, you have to max all of them out first, then you can get the +5 in Outland Engineering since that will be the only one that is not at its maximum.