What would you say are the 'essential steps' in getting a game to work GMless/Solo?

Write the the game as a series of gamebooks written in the "choose your own adventure" style of branching narrative . With skill checks and decisions , for the combats list the tactics the monster(s) will use and their stat blocks and include a battlemap and counters for the fight. Mechanically you can use any system you like for your game book , but something simple like D20 SRD rules should work well and be royalty free.