If you've been following the previous articles and their advice for making a system then by now you should know what you want to make, what the core elements are, and what sort of system you'll be making, whether it's an existing one or a wholly new game. That was all the easy part, now it's time to knuckle down and get to the brass tacks. regardless of how much or how little you're designing there are some steps to take early on. So let's take a look.
1: Imagine. Visualize your game in action, don't worry too much about the exact rules. Instead focus on the players. How do they look? Are they laughing? Strategizing? Are they excited? scared? Sad? Are they cheering or sombre? Do they know instantly what to do or are they agonizing over their choices? Are the players taking turns, one by one declaring their actions, or is everyone acting and adding all at once? How much control do they have over the narrative? What are the /players/ imagining? Are they visualizing how cool their attacks are? Are they thinking about their items or about the twisted monsters before them? Are they looking up references to real world people and areas? Everything about this vision of your game in motion should be considered and remembered. That's your goal.
2: Take stock. Think about what you need in order to get that vision on the table. If they're torturing themselves over which ability to use then you need multiple good interesting abilities. If they're scared or anxious you need a way to put that pressure on them. Even things like if they're rolling dice then you need a die mechanic. If you're using an existing system think of the permutations you need to create. New classes? supernatural templates? New super powers? If you're making your own game then consider what kind of mechanics you'll have to craft. Conflict resolution? A system for dealing or healing damage? A way for players to change the story or each others abilities? Don't worry about how to make it all work yet, instead just think about what you need. It's fine to say "I need a way to get my players to be emotionally invested in characters". Just as much as you might need "A system for resolving ties dramatically". Even if you aren't sure you want it, write it down. It's fine to scrap things later, but it's a lot harder to remember ideas that only briefly occupied space in your mind. Write it down, brainstorm, get ideas out there. If you need inspiration look to your core concepts and make sure you have the intention to create rules that support them.
3:Prioritize. No matter how many or how few things you need it's important to recognize what you need. My recommendation is to take your list of ideas and sort them into 4 categories, I like to visualize them as concentric circles. First look at what is absolutely essential, the things that directly support the core concepts and the things that you absolutely cannot play without. If you have just those things you should be able to show someone what you're getting at, but shouldn't even really be ready for a proper playtest yet. Next consider the things you need before you can do a proper playtest. This includes main mechanics, and things that form the game as a whole. With things in the second category you can run a playtest game, as long as you can control and avoid certain things that aren't done yet. The third category includes the things that round out and complete your work. It covers mechanics that should be covered but aren't critical, this is also likely to include things you hadn't initially considered but realized you needed later, especially things that come up during playtesting. If everything in this category is finished then you have a finished game that you could share, get playtested on mass or even run a campaign with. The Final category is polish, it should include things that make the game better, more exciting, more varied, or that add replay-ability or solve minor problems or clarify things. This category might also include ideas you won't actually implement and is also likely to have things you skipped on during brainstorming.
4: Create. This is going to be the topic of my next article, because there's going to be a lot to cover here, but just know by this point you have everything you need to start making your game, no matter how big or small your project is, you're ready to work it out one layer at a time.
Hopefuly this has been helpful for you, and if you want to see an example of this process in work don't worry, that's coming soon.