Designing and writing a game in a short timeline can be daunting, but with proper organization and a solid effort it is far from unachievable. The key to making a game in a very short time is to do a little bit of preparation and outlining ahead of time; an hour or two of prep makes the rest of the design and writing go by very quickly.
The first step of designing a game is to make sure that you’ve got the core mechanic laid out. Having a core mechanic, like the d20 used for attack and skill rolls in the d20 system, will make any efforts to balance the game much easier, and keeps you from having to write rules over and over. If you are going to have multiple mechanics, which I don’t recommend but is certainly possible if you keep them simple, you should create them all now.
Using a site like AnyDice (http://anydice.com/) is a helpful way to get some immediate feedback on the ways that your game mechanics will work; even if you’re not up on the math that goes into more complex game designs, you can quickly review the chances of certain outcomes with your game mechanics.
The next step is to create a brief outline of the game elements that you hope to include; everything from combat, to character creation, or even simply rules for when people roll should be included here. Lay out an expected time budget if you feel like you must; I suggest planning to have no more than ten hours for a 24-hour game or twenty hours for a 72-hour game; this will keep you on track to finish, as unexpected circumstances will almost always come up during the creation of a game, and quality tends to dwindle if you avoid eating or sleeping (I can attest to this first-hand; Tales of Narvi/Navri over on 1km1kt suffers from it).
Creating an outline will not only make the actual creation of the game a lot easier, it’ll also help you with remembering the bits and pieces that you need. One of my own first competition games was released with combat as a central theme, and lacked any method of dying. The judges noticed, even if I didn’t.
Once you’re in the production process, keep track of how you’re coming on time. It’s not ideal to be a slave to the clock, but you really don’t want to spend four hours on a thirty-minute feature. Make economic decisions with your time, since you will almost certainly have to make a sacrifice or two along the way. If a mechanic isn’t being completed in the time you’ve allotted to it, either come back later or throw in a simple placeholder. It’s better to have a simple rule for everything than one or two very complex rules—not only is it going to be harder to design a complex rule (and write it in a way that people can understand), you’ll also be able to make a more complete game, which goes a long way toward it feeling polished, versus a game with obvious missing features.
Even though you probably have a strict timeframe, it’s not necessarily a great idea to work nonstop. I’ve found that having a little bit of time to ruminate over mechanics and rules explanations after you put them on paper is a good idea. I like taking at least a fifteen minute break for every thirty minutes I work, keeping me fresh enough to write. Keep in mind that you don’t have a ton of time to edit, so pushing yourself until the text becomes a meaningless wall is going to cause issues.
I don’t have a bunch of advice for the time constrained writing/design practices. I know that my earlier efforts at competition games were essentially written in one pass, going across rules as I came to a need for them. This meant that they often lacked components, but they were also rather easy to write. On my more recent entries, and even on larger games, I’ve found it helpful to jump between rules as I think of them and come up with solutions to them, but part of the act of designing is based on practice.
Once you’re beginning to finish up, it’s time to make the game complete. Things like record sheets, which are highly subject to change as the design of your game is fleshed out, should be done here. This is also when I like to go back and add fancy PDF features to my games if I’m going to add them. One of the challenges on this step is to make sure that you don’t accidentally break anything; keeping a backup of an old file is not a bad idea if you’re experimenting with features (something I often do when working on 24-hour games).
The final step of making a game is to have a final look over everything. Make sure you don’t have section headers with nothing in them. I didn’t do this on my first 24-hour game, and it led to some minor issues, and since then I’ve been giving it more time. I give a whole hour to this process, though on more recent games (like Wreck Racers), I’ve found that I really only need more like ~30 minutes. Do any obvious editing fixes and fill in any blank sections that you find. If you’re writing for a contest with a word limit, you should also use this time to edit stuff to be more concise if you’ve run over the limit; I’ve had to do this in the GameChef contest, and it’s not fun but it’s a requirement.
If you do plan to write a 24- or 72-hour game for the Loreshapers contest (or any other game design competition), I’d like to encourage you with the knowledge that it’s certainly something that almost anyone can do it. Good luck, writers!