To answer your second question, most people use the console in tf2, just putting in the command "map [name of the map]," you can start playing any map that's in your files.
There's tons of other useful commands you can use when playing single player, but I'd start with that one.
Publishing to the steam workshop needs to be done in team fortress 2 itself;
Simply pressing this button will bring you to this menu:
In which you press publish new item, and fill in all the information.
Before starting i would
1. Get your map tested by mappers in this community, through our website's map testing server. Milk the benefits dry all you can before putting yourself on the workshop, since you only get one week's worth of a first impression.
2. When ready, collect some nice screenshots of your map to stand out
3. I don't have a third thing, I just cant stand even numbers.
Hope this helps!