Recently Player_03 has stated that he will consider using fan made icons for achievements without them if they are good enough, get to work! You might want to DM player_03#3927 on Discord for feedback and assistance. Just because one is already taken doesn't mean you can't submit one, multiple submissions are appreciated.
Add methods and tips here.
NintendoPanda101: Come up with an idea. Get the needed spritesheets from the beta, take a picture of any level you need, and crop out the part that you need. Open Krita(an art program) and arrange everything you have. Crop and change the canvas size, make sure that the dimensions are the same. Make sure everything is big enough, and see how low you can make the canvas size and keep the overall essence of the image.
Runaway3D: You can use any art program you're most comfortable with. Something as simple as a rough draft can also work. Once you're finished, upload the image in gallery format so that the page doesn't get cluttered. Add the creator's name (which should preferably be you) as a caption.
Anonymous: It doesn't have to be a level with characters, it could be a sign (like Right is Wrong) or line like Figure Skater, or anything, just as long it matches the achievement. Use any art app, but if you want to do one that has characters, get a picture of something(like a level) make sure the app has a magic select tool(or a tool that removes backgrounds from stickers. The run 3 font is called Comfortaa