due to numerous people failing to do this correctly here's a simple tutorial.
Let's say you want to put your altcoinstalks url in your bitcointalk forum profile
Step 1 : click where the red arrow points : 'Profile'
Step 2 : click where the green arrow points : 'Forum Profile Information'

Step 3: Scroll down

Step 4: where blue arrow points : 'Website title:' > Just put Altcoins talks or Just Altcoins
Step 5: where pink arrow points :'Website URL' > put the url of your profile (if you don't know your URL click on help button above)
Step 6 : Orange arrow > 'Change Profile' meaning Save