Log into your Facebook Mobile application on your phone. In most cases, your login information is saved in the application and you simply have to click on it. After you select the application, it should come up on your phone.
Click on the tab that allows you to go to your photos. Scroll to the picture that you want to tag. Then select that picture and bring it up on the screen.
Select the Tag feature from the picture screen. Depending on what type of phone you have, the button could look a little different. For example, on the iPhone, the button you need to choose looks like a rectangle with an arrow in the middle of it pointing to the right. After you click on this button, a menu appears that allows you to choose "Tag."
Click on the person that you want to tag in the picture. At that point, a box appears that allows you to enter that person's name. Once you start to type the person's name into the box, it should appear if you are friends with him on Facebook. After you find the name or type it in, click on the "Done" button at the bottom of the screen. At that point, the person is tagged, and it also appears on his Wall.