First of all, I guess that you have it all connected properly.
On the computer that you want to back up, go to Control Panel > User
Accounts > Click your user > Under related tasks, click "Manage my network
passwords" > Click Add > Under server, type in the computer name that you
want to connect to (You can find this in the computer name tab in System
properties - control panel) > Under User Name, type
*COMPUTER-NAME-THAT-YOU-WANT-TO-CONNECT-TO*\*THE-USER-NAME-ON-THAT-PC* >
Under password, type the password.
Click OK. Now make a blank folder for the backup. Right-click it, and click
sharing. Tick the "Share on network" (And choose a share name under 12
characters) and also tick the "Allow network users to change my files" box.
Now, go to run on the computer you want to back up, and type in
*COMPUTER-NAME-THAT-YOU-WANT-TO-CONNECT-TO*\*THE-SHARE-NAME*
Hope this helps
Harry