How to play solo D&D

The introduction of the 5th edition rules describe the basic gameplay loop of D&D:

1. The DM describes the environment.

2. The players describe what they want to do.

3. The DM narrates the results of the adventurers’ actions.

We can rephrase this into technical jargon as:
1. Content generation
2. Player action
3. Action resolution

To determine how to play solo, we can examine all 3 steps individually and take into account the special considerations of each.

1. Content Generation

In a traditional live game this is the purview of the Dungeon Master, who creates the content of the campaign and expresses it to the players. Conveying the right amount of information is crucial, because if the players know too much then they will not be surprised or challenged, and if they know too little then they will be confused and unable to engage with the game properly. In a solo game, the player and the DM are the same person, so this makes separating the content of the game tricky.

The "Master of Adventures" section of the Dungeon Master's Guide is presented as a toolkit for creating adventures. Many of the options for creating an adventure are presented as tables, and those tables can be used to randomly generate content through the use of dice rolls. 

Randomly generating content through the use of dice rolls and table lookups is a natural method for OSR gamers. The AD&D 1e Dungeon Masters Guide provides three appendices devoted to the random generation of dungeons and wilderness terrain, random determination of monster encounters, and a chapter on random determination of treasure. 

Randomly generating content provides an exciting level of the uncertainty for the solo gamer.

2. Player Action

This is where the actual fun of the game is. This is where the DM asks "What do you do?" Given the world elements, the NPCs, the environment and location your characters are in, your player characters can take their actions. Sometimes the elements of your scene are not clear, and that requires interpretation through a question and answer oracle, or through some guided inspiration like a Tarot deck, but this is where the solitaire gamer can put on their player hat and indulge in the fun.

This is the phase where you put everything together. You can now animate all that content that you previously generated by having your character(s) interact with it all. What happens when they interact with it? Why is it all there? How did it get there? Answering these questions can be useful prompts for writing your character's story.

3. Action Resolution

The 5e system provides a universal mechanic for action resolution, using a d20:

       1. Roll the die and add a modifier
       2. Apply circumstantial bonuses and penalties

       3. Compare the total to a target number

The thresholds for the target number (called Difficulty Classes or DC) are described in plain English, so this system can be used as-is for most in-game task resolution. For the solo gamer, simply determine what you think the DC of an action is, then roll the d20 and add modifiers to see if you succeed.

Most of the action resolution rules in 5e are split between player facing and DM facing rules. The chapters devoted to "Using Ability Scores", "Adventuring", and "Combat" are all technically player facing, while the DMG chapter "Running the game" is for the DM. Simple dice mechanics can be used to cover almost all types of results, and for the rest of the unknown results it is advisable to use some kind of oracle, such as the Mythic GME.

Most solo oracles are devoted to the Action Resolution stage of gameplay. The Mythic GME uses the Fate chart, while other oracles use simple yes or no resolution. A simple d20 roll can also be used as a "yes or no" oracle as well.

Once all success or failure results have been determined, the solitaire player can return to the Content Generation or Player Action stages of the game, to keep playing and continuing the loop.

These three steps cover the "how to play" section of solitaire role-playing, and to know "what to do", the player must adapt the "Three Pillars of Adventure" to solo play, which I discuss further in depth here: https://farooqsgaming.blogspot.com/2020/05/the-three-pillars-of-d-solo.html