Well, for one, you should join the REMS discord server if you have any specific questions.
The first tool you'll need is the Maniac Editor, or literally the only level editor for Sonic Mania in existence. The next you'll need is some sort of graphics editor to make your tilesets. Then if you didn't choose gimp or graphicsgale as your image editor, I would recommend downloading either of those. Simply take the image file you want to save onto and drag it into graphicsgale/gimp and then copy & paste what you want to save onto it and save in graphicsgale/gimp. (If you want a suggestion, I'd recommend GraphicsGale due to load times and ease of use.)
That should be it. For extra stuff you'd want to use a palette editor like ManiaPal and the RSDK Animation Editor for making new platform frames and making everything bigger, etc.