Sorry for the poor formatting, I'm just copying out of my Google doc for easy of use!
Writing and Style
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In this Chapter, we will look at the act of writing itself. By the end you will know some habits to keep and pitfalls to avoid. The second part of this chapter focuses on style and details how writing in a particular way can help the GM and players pick it up.
The first rule of writing an RPG is to keep writing. Do not edit until you have a full first draft of the game. The first draft will be poor but you need to have a complete game before a proper edit can occur. If you get stuck on an area, make a note in the document (I use a load of Xs like 'XXXXXXXXXX' to make it easier to pick the notes out) and move on. Writing a roleplaying game is a little like writing a novel, some of the ideas included here are applied to both and you can plunder novel writing resources if you get stuck.
Getting it finished
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Any large project requires dedication to complete. Initially, you will have a fire and passion of enthusiasm, which will last about one third into the project. If you manage to force through that barrier, the next drop in enthusiasm comes at two third through. If you can get through those then the chance of you completing is extremely high. Here are some habits and tips to help you push through those barriers.
Set a deadline to have a 'test' version of the document. Stick to it. Produce whatever you can by that date. Whatever you have at that date, release it to the community - even if it is far from finished.
Schedule small releases to the free RPG community. Release small, release often.
Collect feedback from the community but don't edit your game right away - wait until it is finished.
Don't re-read the game until it is finished.
Set aside a time each day or week when you sit down to write. Do write outside of this time but never miss it. Use a calendar to set a regular appointment with a reminder to send you an email/text/tweet.
If you have control over the computer you're writing on, set up a new user account that does not have access to games and puts parental filters on games/RPG forums sites to reduce the distraction.
Use whatever time you have, great progress can be made even in a half-and-hour lunch break or while the newborn baby is sleeping.
Schedule what TV programs you are going to watch and watch only those. Never channel-hop.
Open a dialogue with friends and family (non-gamers too) about what you are trying to achieve. By talking about the game, you will find it easier to keep motivated. They might also enquire how it is going and that acts as a softly softly pep talk. Show them the work you have put in.
Write the game in any order. Later in this guide, you will learn about ways to organise your game. Organise it last, write it first.
[Optional] Play appropriate music to the genre you're writing in for inspiration. Soundtracks to films that inspire your genre are useful.
Good practise
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Write these best practise tips on post-it notes and stick them near the place where you write.
Never delete text forever - when cutting a section, copy into a 'scraps' document, label it and leave it for later (I do this with graphics too).
Do an off-site backup your work weekly. Either upload to a free file storage (such as Google Docs) or put on an old USB
thumb drive and hand to a friend or put it in your desk at work/locker at college.
If you are stuck finding a name for something, use Thesaurus.com to help find similar words.
If writing starts to slow, move onto the next section.
Writing rule Examples
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A good example is essential to making your game playable. Examples should be both compound and independent. Compound examples are where one example leads on from a previous example, for example, if you describe an example character John Smith with a Strength of 9 in one example, John Smith should have a strength of 9 in all examples thereafter. Independent examples do not rely upon previous examples to be understood. A single example should be enough to demonstrate a ruling. Remember that the GM uses the book as a reference, so lean towards independent examples or repeat the important parts of John Smith.
A good example takes a small part of the rules and demonstrates it. Larger examples can build upon the simple, atomic examples but be sure to include both. Atomic examples are good for reference, longer examples are better when the book is read through at first.
Actual play examples can be useful but be careful to keep them curt and to the point - you do not have to write precisely how people speak. If you have particularly tricky parts to your system, include more than one example. It is unlikely that your audience will be completely new to roleplay.
Style
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Poor style can make your game inpenetrable. Good style can make a complex game appear simple.
Write the book the way it is supposed to be played:
Optional rules are fine, mark them clearly as such.
Do not load down the game with 'The GM can ignore this if he/she likes...'
Avoid a chatty style of writing, it adds words and does not help the reader.
Justifications of why a rule is prefered over another belongs on a website.
Describe your game objectively and compare it to others if you must but avoid 'marketing speak' regarding how epic and wonderful it is.
Avoid elaborating in too much detail on a part of the system which is not core to the concept.
Do not add anything to the RPG that is not going to enhance the concept. If you have an idea for a tangent, write it in a notebook and use it later.
Avoid including your design process, that is best left on the website or internet forum.
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All feedback and comments welcome!