Here, I actually just made a similar post on the fumo thread that might help you
Follow this general process, think of the character you want. Then, think of what fumo has an expression that matches what you want in a fumo of that character. Buy them, as for other supplies, I use knitted fabrics for clothing due to its softer and stronger nature, alongside it oftentimes not fraying when cut. For hair, I have used minky, blizzard fleece, and anti-pill fleece, and all of them are rather good for fumo hair. Much like clothing fabric though, just go to a fabric store, find the colors you need, and then just start feeling different fabrics to see what feels good to you and looks good at the same time. For painting eyes, which is often necessary, its actually quite easy, just take it slow and remember youll probably need to apply multiple layers, use an acrylic paint, and youll be fine. Removing the clothing of the preexisting fumo is easy, there are 1-2 anchor stitches on the belly button and directly opposed from that on the back, snip those and you can either slide off or bisect the clothing. The hair its really just giving a fumo a haircut down to the scalp with a pair of fabric shears. Last thing, making a fumo is not the hardest thing in the world. Take your time, be patient, and itll turn out fine. There may be times throughout the whole process wherein you think its turning out terrible, it doesnt look good, etc, for my miyoi >>23256440 I thought that up until I was stitching on the last bits of the skirt. When incomplete, do not let its current look make you think it looks terrible. Lastly, dont be afraid to mess up, making clothes you can easily redo, same with hair patterns. Removing the hair is hard to truly mess up on. and as long as you take it slowly, eye painting should bring you no trouble.
If you have anymore questions please ask
Oh yeah, one addendum I wanna make here. Expression is the most important definer you should use as a custom base. However, it is possible to change it, I don't know if anyone else has done this, but for my Ruukoto, I took the :< expression of Sakuya and made it a smile, but pulling out the previous mouth with a needle, uplifting the thread and removing it, then hand stitching a new mouth. Again, it kinda sorta isn't really a thing that people do, and I have no other documented cases of people doing that. So that one you'd largely be going into uncharted territory, and I would only say do it as a last resort, or in the case of something like my ruukoto, if the base otherwise is literally perfect.