Nameless Sprite Editor allows you to put in an offset for a picture, and draw different tiles. However, I just use Microsoft Paint, and A-map. Click the purple puzzle piece in A-map to open up the block editor. Going to picture->save tileset 1 will save the top tileset, while save tileset 2 will save the bottom tileset. After you save the tileset, you can open it in paint, and draw whatever you want. Then load the sprite into the block editor by going picture -> load tileset 1 (or tileset 2), and selecting the edited tileset. The edited tileset has to include the same colors as the pallate, or else it won't load.
If you don't understand this, then I'd wait until you have more experience and try again.