First step is to join the guild. The main npc at each guild can get you started. At that point you can start to craft. When you craft something there's a chance that you'll get a skillup just like with combat skills(weapons, evasion, etc) You can check your craft skill by going to menu>>status>>craftskill
at lvl 8's: 8, 18, 28, 38 etc you can increase your rank by trading a specific craftable item to the guildmaster
at lvl 10's: 10, 20, 30, etc your lvl is capped and you will not gain skillups until you increase your rank
if you go to the links thread that's in this forum, you should find links to all this information in much greater detail