It should WOW me! Seduce me! Lead me in and hook me! You want to keep them engaged, interested and most importantly not skimming to the end just to read the damn rules (though thems be important too) personally I like to make an interest check with some kind of fancy header image and 'title' image for example a long while ago I used (and pilfered) [url=https://www.roleplayerguild.com/posts/5241145]this kind of format[/url] for a D&D game I ran, simple and short it is enough for a game of D&D like that.

Or you could go for a big long post, filled with words and words galore! Not too many, but not too little. Enough to be digestible and enjoyed in full.