To the best of my knowledge there is no preferred format for adventures. I would suggest you look at some of the previous Rifters that have adventures in them. 47 has The Fall of N.O.R.A.D for Rifts and Chaos Earth. You can check out the Rifter Index for more examples of adventures.
Just use what most books do and have a beginning, middle, and an end. Make sure any relevant N.P.C.'s are stated out and have fun with it.
Correct, there is not a specific format. That can cause a little confusion, but it is also an advantage for the writer to be able to best present material as the specific adventure requires. For example the first adventure of that NORAD Duology was organized by room and by day, inappropriate for most other adventures.
Just follow the suggestions in the submission guidelines on the main site (proper abbreviations and such). I strongly suggest getting a second pair of eyes proof read the material before submission.
Currently recruiting for Beyond the Supernatural games in 2019 which I am running on Discord: voice, text, and play-by-post. Here is the non-expiring server invite link: 418BQSLG
“All would be well. All would be heavenly— If the damned would only stay damned.” -- Charles Fort, The Book of the Damned, 1913
I like the style the others have suggested. I am not a fan of "Hook, Line, Sinker" aventures. Other adventures I like include the PFRPG Yin Sloth Jungles material. I also enjoy using random encounter tables, like the ones in Mutants in Avalon (After the Bomb) for example, to flesh out the story while travelling from place to place.
I bought the Rifters 31 & 32 (or was it 30 & 31) on auction for the castlerake adventure. Check it out as an example.
Rifter 52 Cannibal Magic Rifter 55 The Ancestral Mystic P.C.C. Rifter 59 The Lopanic Games adventure "The Lion, the Ditch & the Warlock". Illustrations to this adventure can be found here. Rifter 71 & 72 Double Issue Ninjas & Superspies adventure "On a Wing & a Prayer" Rifter 80 Masters Unlimited