A custom character is a big file plus a few fragments and quite an undertaking. It is also one that many would be reluctant to invest in without some sort of promise that a work gets published that uses it so having a lot of progress to show might make artists feel better about volunteering. You may find yourself having to do it if you are not yet at that stage, or put it on the list of things to do later. Instead of trying to approach it all at once or thinking it must be done before you do too much - a very daunting thing - my recommendation to you would be to break it into several steps.
Skip right to step 3 if no non-native setup is needed (i.e. everything SMW has as default behavior is perfect for you,) and remember everything about the character graphics implementation can be considered as late as the very end of your project development.
1) Is my character going to have different physics/powers/size? Patch in the code that will actually represent characters' abilities, especially physics, because your entire hack's design will depend on physics. Size will impact how tall your corridors should be, e.g. if your character is always going to be two tiles high then your design liberties will be different than if it were 1 tile high. Then, if any powers are necessary for progression or change how the character interacts with the level in a manner that you may get softlocked or forced to lose the power before progression, you will want to handle that accordingly. This is the most essential part to have done of your character before you get too far into your hack, if it applies.
2) Does my character have a health system (and do enemies have variable damage?) It can be added much later into development, so that you can distribute healing items to rebalance it, but should not inhibit designing levels.
3) Do I have my character design laid out?
- Sketch a few poses and expressions that suit the actions being performed. It doesn't have to be detailed or exact, just a rough representation of what you are imagining.
- Consider the style of your hack. Unless you are going for something with super realistic proportions, your character's head might be close to or even half the height of the full sprite since the face and hair/headwear is a predominantly defining feature of a character compared to the rest of its body. Look at Mario and Final Fantasy for examples of what you can expect to fit into this space and what proportions to keep them expressive and identifiable.
4) Set up your work environment.
- Back up GFX00 and GFX32, as well as SMW's default palette.
- Set up roughly what you want the colors to look like for the new character in a copy of the shared palette on palette row 8, but bear in mind that SNES has less color depth than what YY-CHR can show. Every R, G, and B value that is a multiple of x08, but starting with 00, will have representation in SNES. Lunar Magic will round the color if it is not valid to one that is valid. Bear in mind that color 00 is transparency and changing this will affect nothing nor show a color, and 01-07 are used by other sprites. 08 through 0F are strictly your character unless you are redrawing and recoloring other sprites, which can also risk being altered if your powerup changes your character's color.
- Determine whether you want to use the existing tilemap or remap the character tiles. Remapping offers the benefit of not worrying about playtesting for alignment issues so much, and giving you the freedom to arrange tiles as you please. You can use SMW Player Tilemap Editor to easily preview and save changes to the tilemap, and you can use an infographic or a control GFX00 and GFX32 in another tile editor window to see where poses are if you intend to leave tile mapping alone and still cut + paste tiles to see how they align. Feel free to open yet another instance of YY-CHR as "scratch space" to draw a full-size 16x16/24/32 frame if you are more comfortable cutting and pasting completed frames over old ones. So again:
* Copy of GFX32.bin for your new character
* Copy of regular GFX32.bin for reference OR tilemap infographic from the Data Repository forum (if needed, not so much if you are remapping everything)
* "Blank" 4bpp SNES window to draw in if cut+paste is preferred.
If your OS supports multiple workstations, I would put them all on their own station apart from your web browser window or whatever to keep it organized.
(Worry about GFX00 last IMO. In the SMW Data Repository there is a nice reference for which poses these tiles appear with. You may find yourself not needing GFX00 hands, feet, and cape parts, in which case you can delete those tiles to free up some space.)
5) Start drawing. Pixel art is very different in practice from hand-drawn art, since it's more about easily-changed placement of these single-color squares to build a picture rather than steady strokes, pressure, and precision. If you search "Derek Yu's Pixel Art Tutorial" - I have sworn by this one for anyone learning pixel art because it is so quick, simple and clear with good examples, although you will find as you get more experience, you may not need so many steps to flesh out what you are making, especially if it is a small sprite. Plan to work on-and-off on this for several days if you invest like a half hour at a time, because it is not a one day project. I can't see it taking less than 3 hours for even the most committed spriter for a full GFX32.
Insert your work so far, including the shared palette, and change the other 3 Palette 8 rows accordingly.
6) Play test to verify alignment and to see if any necessary frames were missing. Grab powers to see if the four palettes you are given for the character are alright or need more adjustment. See if anything you modified in the palette affects sprites. Using Tweaker, you can remap the palette used by most sprites away from 8 if they are negatively impacted but look good with other color sets. You don't have to finish your character before you playtest, but can preview what you have done so far.
It will come together in time. Just remember. Prioritize, organize, and work patiently. If any character already uploaded to SMWC looks similar to what you need, e.g. you have a Mario fan character or Mario universe princess that is similar in a lot of ways to Peach but is not hosted, you can also save time by tweaking color and less pixels than a full revamp, but more likely than not your ideas will be different.
Just look above you...
If it's something that can be stopped, then just try to stop it!