Insert as an edit notice for a user page as {{subst:Editnotice userpage}}.

Example:

To insert, go to your userpage. Click the 'edit this page' button at the top. When the editing screen comes up, there should be some small words on top of the edit box that read 'Page notice'. Click on this, then create a page called User:YourName/Editnotice. If you don't see the 'Page notice', type User:YourName/Editnotice in the search box (which should not bring up any hits and allow you to create a page of this name. In both cases, you can now paste {{subst:Editnotice userpage}} into the page and save it. Then, whenever somebody clicks to edit your userpage, there should be a box reminding them.