Use W and S to move your character up and down.
Your shield will be held automatically, so no need to hold down secondary fire.
If there are 0 players on a team, a simple AI will spawn. However, the AI is very basic and having 2 players is much more fun.
You can edit a few settings in the Workshop Settings. These are already set to default values that are intended, so adjusting these may negatively impact the gamemode.
Disclaimer: When there are server issues with the workshop, players will sometimes not spawn in correctly. Restart the custom game until both players are in the correct positions. I will look to fix this in future versions if the problem persists, although it depends solely on Blizzard's servers working properly.