Now that you know all about Modkeen, start drawing your art. I'd start with designing on paper the basic settings of your game. After you've done that, draw some basic background tiles. If you like the way it's turning out, continue with drawing more detailed, special background tiles. try to be innovative, don't make everything the same. Most of all, make sure the tiles fit, and have a reason that they're there. Don't just have a picture of a cow in the middle of nowhere. Next, you need to start designing your enemies. You always want to plan out what you want on paper first. When you're ready, you can start drawing on the computer. Always make the outline first, and build off from there.
Time to download TileInfo. TileInfo has a number of functions that you'll need to know. Read up on them here.
Now that you've finished your graphics, its time to make a choice. There are a number of level editors out there that you can choose from. The only two worth using at the moment are iKeen and KeenWright. Read this to learn how to use KeenWright.
Read up on CKPatch here.
Now that you know about CKPatch, you should do your TileInfo work. Remember not to save a TileInfo file with the same name as a patch file of yours because TileInfo automatically creates a patch file with the information for patching. You don't need this, because the Patchotron, which you are about to learn about, can do this. Read this article now.
Now for the finishing touches: download Fin2Bmp and read how to use it. Now you can change the ending screen!
You're done! But you have to advertise your game, and people will want to see screenshots before they download it: Download Screenthief and read how to use it.