DIYRPG Campaigns and Adventures

I think the best thing to do is to keep it simple and basic.

Nothing too elaborate with many moving parts. No big twists. The goal is to introduce players to the basic principles of playing RPGs, and to let them get familiar with the most basic of the rules of the game system.

Since it's a one-shot, you can start with pre-made characters the players can pick from. If you have 4 players, make it 6 or 8 PCs to choose from. When running D&D 5th edition, make it simple characters like human fighters, halflings rogues, or elven wizards. Dragonborn paladins and tiefling sorcerers can wait until later when the players have had some time to learn what those things even mean. Don't have them choose feats when they don't know the game yet. Either by picking them yourself for pregen characters, or not using them at all. Similarly, if the players do make their own characters, let them assign attribute scores from a fixed list instead of distributing single points between them. Picking which attributes to make highest and lowest will be challenging enough at first. Having to set the specific values makes things more complex than it really needs to be at that point.

Same approach goes for the adventure itself. Keep things simple. Getting a hang of the dice and the turn sequence will be plenty enough workload for new players to start with. Tell them right from the starts where they are supposed to go and what they should do there. But I would send them to do something like "go to that ruin and search for that treasure", since that is a goal that tells them what they are meant to accomplish, but leaves it completely open how they will do that. And then let them get away with most things they want to try. If something sounds like it could work, let it work, even if it's not what you planned to be the solution. We want the players to get in the habit of trying out things for themselves, not make them think that there's always one correct solution they will have to guess. Make them roll when a skill check would be appropriate, but keep the difficulty low so that they will succeed a lot and only fail rarely.

Make an adventure that practices the procedures for using the rules of the game and keep things straightforward to minimize confusion about what's currently going on. Once the players know how the game works, then it's time to introduce them to hard choices, difficult challenges, and unexpected surprises.