1. While you're at the title screen, click "Create Lobby"
2. Make sure "Make Private" is checked, then click "Create Lobby" once more
3. Now make sure either "Medium" or "Large" is checked, which one you pick doesn't matter. Click "Start Lobby" and click it again when another prompt shows up. The game will create the world for you even if there is only one player in the lobby.
4. Join a team and click "Start game".

You did it! You are now free to do whatever you want without worrying about the prospect of other human wizards eliminating you. Craft, trade with gnomes, gain legendary loot, practice casting spells and using items, the world is yours. The game will essentially never end. As long as you captured the flag in your own base, you will respawn when you die and the game won't end. Even capturing the enemy base won't end the game.

Tip - You can also select Colosseum from Step 3 and you'll get the same experience except that you're trapped in an arena, but running around to smash the pots will give you spells/crafted items/soups and doing this be the fastest way to get many of those although all the pots won't contain EVERY item in a single game. You can't choose what items spawn in the pots either.