It's extremely easy!
Navigate to your friends profile - you can do this by searching for your friend in the search bar or by clicking on their name from any recent post of theirs.
Once you are on their page you can get started by typing in the "Type Something" space. Post like you would on the home page, but now it will appear on their wall! You can post a photo, a message, a link - you have the possibility to make it whatever you would like!
To have full capabilities - you must be a Full or Creator Member. However Free Members can still post some items!