I've had a long week, because I've been basically moving on high gear for the past 5 days and I'm like exhausted as woah. So some brainless chatter.

Why solo D&D?

Have you ever solo'd an MMO? Played solitaire board games? Played chess against yourself? Played any form of "multi-player" yourself?

There you go. If you enjoy a game, you wanna play it and you don't want to have to wait until you can wrangle 3-5 adults into setting up a schedule to play with you. Hence. Solo D&D.

OK, but why D&D?

Because it is the biggest TTRPG on the market. There's a lot of published adventures for D&D and once you figured out how to play D&D by yourself, you'll probably have a good idea of what kind of rules are fun for you versus what kind of rules you don't like. And you'll be comfortable running through any kind of system and hacking it into doing what you enjoy.

So, let's get to playing some solo D&D.

Step 1: Get your playing stuff.

Pen (or pencil) and paper. Or replace with a digital spreadsheet. A set of standard polyhedral dice. Or replace with a digital dice roller. Find an adventure book (I recommend here for a pretty good collection).

Step 2: Build your character (and party).

Most D&D modules are written for an average of 4 players (usually spanning from 3-5). To play solo, create your character, then create 3 sidekicks. Use the sidekick rules to create a sidekick of each type (expert, spellcaster, warrior). Here are some general things to consider when picking creatures for a sidekick:

For the expert sidekick, definitely pick a creature with multiattack to ensure more hits. I find Hadozee Warrior to be pretty good, especially with the glide trait and 30-foot climb speed, meaning you can easily test for pit-traps.
For the spellcaster sidekick, Sacred Stone Monk is a versatile pick (if only to get better AC boosts), and it has a reasonably decent wisdom stat that you can pick the cleric and druid spell list for very good support skills. Alternatively, the Changeling is a strong option for access to the Wizard spell list if you need more Wizard spells and/or make for a good Charisma caster if you'd rather just rather have an eldritch smite bot.
For the warrior, if you aren't planning to go past level 5, pick a creature with multiattack, since the warrior sidekick isn't going to get the extra attack feature until level 6. However, if you are planning to go past level 5, pick a creature with a strong single attack, seeing as the extra attack feature becomes useless if the creature already has multiattack. Valenar Steed or Worg are both good options that can also double as a mount during travel. Fist of Bane is nice for just pure damage.
Consider giving every sidekick an origin feat (two if they're human) to help bolster their variety. Recommended feats include: Tough, Alert, Lucky, Magic Initiate (if your sidekick take this one, definitely choose Wizard's Find Familiar for the level 1 spell option).
If you pick creatures that are less than CR1⁄2, boost their HP up to comparable HP for a CR1⁄2 creature. If they're getting hit a lot, give them more HP.
Regardless of what skill proficiencies you pick, make sure Perception is one of the options. You will absolutely need everyone to have good Perception as you go through the adventure.

Level your party to at least Level 3 before starting the adventure.

Even if the adventure module states that the adventure is written for levels 1-3 or some such, still start the party at level 3. The power scaling is pretty severe. If you don't want to give yourself too much level advancement, give yourself two or three magic items to cover the shortcomings of your single PC. As a rule, keep the party at least 1, better 2, levels ahead of the curve to make up for the fact that you only have a single full fledged PC's toolbox.

Pick an adventure then start running your party through it.

Don't bother with reading ahead. Write down your decisions before resolving the action so that you can't cheat yourself out of the surprise. Here are some things to to make sure you hold yourself to the roll result rather than "cheating" your way through (although you are playing a solo game, so cheat if you want to):

If you're not sure the party would have looked or searched in an area, simply assume you would and roll to see if you beat the DC.
In social encounters, roll one of Persuasion, Intimidation, Performance, Deception to see if your character succeeded. It should be pretty intuitive which trait makes the most narrative sense. Don't be tempted to constantly pick only skills in which you are proficient.
Roll Sleight of Hand when lock-picking even if the adventure's direction says to roll a straight Dexterity check.
Write down the result first, then check what you get to resist the urge to infinitely re-roll until you succeed. Hold yourself accountable for the failure and keep progressing from the fail state.
Failure does not mean there's no way forward. If the adventure doesn't give you an option, just make up something that's narratively sound and move forward in the action.

Run combat according to the rules as written, but keep the action moving.

You will probably forget hand availability, spell casting components, and a myriad of other rules. It's fine. Keep going. The more combats you go through, the more the rules will become familiar. Don't worry about getting it "right". Once you've looked up the rules enough, you'll start remembering how conditions and grappling work. However, don't try to circumvent the rules as they're written. Interpret the rules in the way that makes the most sense to you and tweak it only if it gets in the way of your fun. Since only you can hold yourself accountable, running the rules as written becomes part of the challenge.

Re-spec if things are not fun.

If things aren't fun, change out your character build. Change the subclass or play a completely different class altogether. Change the spells on your spellcaster sidekick. Give your warrior sidekick a magical weapon. Pick a completely different creature to be your sidekick. Give yourself permission to shake things up in the middle of things just so you can get better results if that's what you want. Again, the game is as easy or as difficult as you want. There's no reason to make it harder if it's not fun. But similarly, if you prefer to play a meat grinder, then simply make the monsters deal more damage or hit more accurately.

Modify criticals.

If you want to make combat go faster, use the modified critical hit: instead of rolling the damage dice twice, assume max base damage then roll damage again, adding both values. For example, if the damage is 1d8+3, roll 1d8+3 and add it to 11 for the critical damage.

Aaaand, that's all folks. Enjoy your solo campaign!