I thought I'd like to write a few things about rule writing that I've learned over the years through painful experience. I've never found a set of rules for any period that I liked all the way through. So, like a mad Dr Frankenstein, I'd take the parts I liked and made up the rest myself (sometimes also taken from other rule sets).
Although it is nice to bring to life a period you have an interest in, sometimes the creation can be a bit of a monster- especially when you have to get back to a set of home-brew rules after a period of time away from them. So I've developed a system (of sorts) to aid me. The key is to be organized.
First, you need to be able to type the rules up so you can print them out. I use Microsoft Word. I have a directory off my documents directory that is called "rulesets" which I also subdivide into eras (e.g. Ancients, Napoleonic, WW 2) then the directory that is the name of the ruleset. In each directory for that ruleset, there will be a Word Document (the rules), a PowerPoint Document (the charts), and an Excel Document (where I create the charts) as well as any other sundry things (pictures, etc).
I do my rules using the bullets and numbering set to outline mode. This way each section has a number and any rules under that the corresponding numbers.
My intro is usually an overview of what the ruleset is, as well as its scale.
My 1st section is always the turn sequence, in as much detail as is necessary for me to follow down it during a game (usually it becomes second nature again after a turn or two, depending on how long I've been away from that particular game).
The 2nd section contains a section that contains definitions. This is an important part of the document, that you will be adding/subtracting from as you refine your ruleset. Standard terminology is a great boon to good rule writing. For example, if I use the term "Good Order Unit" in my rules, I know that it means "A unit that is not marked as fatigued, disordered, or routed". In the long run it tends to create more concise rules that are more consistent.
The 3rd section is used to point out any special procedures needed to play the game (e.g. For a particular ruleset I use wooden blocks set next to units in order to keep track of actions that a unit has done for that turn. If the unit has attacked, the block is placed standing up. If the unit is finished, the block is placed lying down.)
From the 4th section on, tend to follow the turn sequence, each new numbered section detailing a new part of the turn sequence.
The last section(s) are special rules, "chrome" and anything not already covered above.
Do not forget to create the header/footer for the document. I use the name of the ruleset as the header, and for the footer I use "Page # of #" along with a date. This is very important when you wind up with several versions of rules and the pages get all mixed up.
For charts, I use Excel to create the tables I need, then cut and paste them to PowerPoint (giving them a nifty shadow under them) for printing. I try to keep the tables to one sheet of paper, although sometimes its not possible. Clarity is the key !
After you print the rules and tables out, playtest, playtest, playtest. Keep notes of what works and any inconsistencies that crop up. Note potential solutions. The key is to follow the text you've written- if it doesn't work- rewrite it so that it does.
Eventually you will tire of that period and ruleset and move on to another- don't worry ! When you decide to come back to that set again, you will have all the material needed to get it up and running again with a minimal of head-scratching. Enjoy !