Go into tf2 with your map already compiled into a bsp and with all assets packed into it. Click on Workshop to get to the next menu.
This is were you can submit your map to the workshop. You want to go to "Publish a new item" and set the title and description for your map.
Give it a Preview image which is usually just a screenshot of the map. Then click on Maps and upload your .bsp.
Click submit and it should show up on the workshop.
After submitting you should be able to edit parts of it on the page were its been submitted.
And theres also an old thread that I found on the site from Tyler in case I messed up or missed anything.