You could use a registration plugin (many freely available) rather than the core WP registration feature. Many of those plugins will auto login the user and then offer you an option to redirect the user to a specific page. You could setup it up top redirect to the where the user can add their entry.
You could also add the instructions to the welcome email the user receives after registering. Here’s one such plugin to all you to do this. There are others, this is just the first in the Google search results.
Hope this helps!