You can just buy some 40 volume creme from amazon or a hair salon. I used it on my NES 2 years ago and it still looks like its brand new. Heres the link to the stuff I use https://www.amazon.com/Marianna-Super-Star-Creme-Developer/dp/B00FKVYCHQ/ref=sr_1_2_s_it?s=beauty&ie=UTF8&qid=1499263024&sr=1-2&keywords=40+Volume+Creme
You can just use hair developer. The main active ingredient in RetroBright is hydrogen peroxide. There are much easier ways to get hydrogen perxoide gel than making it yourself. Here is some on amazon
You still follow the same process. Paint it on the part, wrap it saran wrap to keep it from drying out etc.
honestly, a higher concentration wouldn’t be bad at all. Mix the 40 volume developer with one part water and you’ll have a 6% solution. Twice as strong as regular drugstore peroxide, so more efficient, but won’t damage the bone. Then you also have a higher volume
When looking for developers, look for one that says clear. This means liquid. This is the one I usually use, it gives good results
I'm sure doing a retro-bright technique will work.
You can try putting it in a sandwich bag with enough hydrogen peroxide to cover the pen (Remove your tips). Then set it out in the sun rotating it to expose all sides. But the concentration may be too weak, worth a try though.
I've used this on plastic and it works perfect, it has a much higher consecration of hydrogen peroxide: https://www.amazon.com/gp/product/B00FKVYCHQ/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&th=1
Smear a little over the yellowed plastic (Use gloves), cover it with saran wrap then let it sit in the sun for a few hours rotating it once in a while to expose all sides.
its pretty simple. use this and put a this layer of it on the soles use cling wrap to cover it and leave it in the sun for for a hour or two while checking on it from time to time. you got to make sure that the sun is hitting strong with no clouds. that will mess up the process. you also have to make sure that the brush or you could a tooth brush that has clear bristles cause the color comes of cause the creme is bleach, its pretty easy to do