It might be invisible, so right click inside "RAW Paste Data", and click "Select All".
Right click inside "RAW Paste Data" again and click "Copy".
If you're having trouble copying it, click "get" for a download of the character.
Now head over to the change profile name screen from your friends list window.
If you have the username you want already set up, simply paste the character from the pastebin behind your username. Pasting it will add the character, but you won't be able to see it. In the screenshot below I've put a marker character displaying where you should paste it.
NOTE: When you paste the character you won't see anything. The screenshot below shows a symbol just so you understand where you should paste it.
Now go ahead and hit save. You'll see something like this in the chat:
Your alias history now shows two identical usernames. Go back into the change profile name panel from your friends list and add a second pastebin character before your username. Again, this change will be invisible but I've added a character in the screenshot below to show where you should paste it.
Save it, and repeat this step by adding an additional pastebin character each time. After you make a few username changes, Steam will prevent you from changing your username for a bit. The wait isn't very long, so don't worry.
After making ten unique changes using the pastebin character, your whole alias history will display the same username. Be sure to change your username back to the version without the pastebin characters when you're done!