From UO Second Age
From the Second Age Play Guide, printed in 1998:
Double click on a pile of kindling (kindling can be bought in towns or gathered in wooded areas) to make a fire. Your skill will be checked automatically, and if successful you will receive a message that your camp is secure. To log out from a camp, use your Bedroll in the vicinity of the campfire.
Bedrolls can be purchased at the Provisioner shop or by selecting the Camping skill during character creation.
Kindling can be gathered by double clicking on your dagger, then targeting a tree.
Using the Skill
When you get the message about your camp being secure, drop your Bedroll, double click it to open it, and then double click it again. You should get a gump confirming you're about to instant log out. Click Okay. You will get a "Connection Lost" type of error, and your character is now completely logged out.