Get a program like AdvanceMap and grab the ROM you want to edit (make sure you remember exactly which version of which game it is, as you will be required to communicate that to everyone who wants to play it instead of sharing the edited ROM itself, you only share an IPS file). Take those maps and draw new maps, and start learning how other things work. You'll need to redo map connections (you don't want your Route 1 northward replacement merged with a tree wall on the south end of another city), you'll need to set where players can move (so they don't climb on top of a rock or jump up an express route), and you'll have to set weather and music. You can change how tiles work in the included block editor (such as making a tile a form of ice, and you slide across without stopping, or setting to hide behind the tile like a house roof, etc.; this and movement permissions are the majority of the work I'm doing on Light Platinum, actually, as there are a lot of messy errors like this). There's a bunch more you can do there that I won't go into here as that's better for a
ROM Hacking forum tutorial or Q&A.
I wouldn't start putting in trainers and everything until the time comes that you're done most of the maps. Some (many?) disagree and would build one map at a time the entire quest, but I think it's better to map out the entire game and have your plans on paper. At this point, you still have not shared the hack with anyone because you have no Pokémon, no Trainers, no story, and not even a choice of first partner. Once you have the entire region mapped, figure out where you're starting and set up the introduction area (remember all the maps for inside buildings and gatehouses everywhere, too!). Start numbering your Routes in whatever fashion you want (i.e. welcome to Route 5001), and they don't have to be sequential or even numbered (Sienna uses examples like Colt Pass, Sian Pass, etc., though I like numbering or lettering - such as Area B, Area U, etc. in another hack - better). Trainers are actually scripted through another tool which you can use at that time to start planning those and items, and signpost readings. At this point, you are beginning to build your story and getting to the point that you can start sharing your work (though if you want people stopped at a certain point, make sure to script an entire row of tiles stopping them from progressing and turning them around while still seeing the few tiles beyond the scripted stop).
And that's most of it. Figure out Gyms, figure out an E4, and make a good story that makes people want to play to the end.
