That is somewhat expensive, and you almost certainly have some variation of two thirds of the kit already…
I recently ran out of cold blue, and bought this ebay item to replace my stock: **LINK**
£19, but 500ml is a massive amount and will last me years. I've done a few parts with it and it is as effective as any others I've used.
You need some kind of degreaser, ideally a water based one rather than a solvent or a light oil etc. I use Screwfix no nonsense degreaser: **LINK**. Be sure to read the instructions, and dilute appropriately, as you only need a tiny amount. 5l will last years. I also use this as my default ultrasonic cleaner solution.
Clean the parts well, with a water based degreaser. I tend to clean parts in an ultrasonic cleaner if they'll fit.
Don't touch the parts with bare hands again – you should be wearing gloves when using the selenium dioxide anyway, but also avoid greasy fingerprints.
Rinse the parts off in clean, hot water – close to boiling ideally as that both heats the parts and allows them to dry quickly. Hotter parts react with the solution better.
Allow the parts to dry – don't try and speed this up by blowing oil over them with a compressor, as that will defeat the object of the degreasing step.
Decant a small amount of the bluing solution – maybe a capful, probably less. Don't dilute it, and never return used solution to the original pot.
Don't try and submerge the parts, that really doesn't work. Use a toothbrush, and rub the bluing solution into the surface. I don't know where I got this particular bit of advice from, but it is pretty much the most important step in getting a good finish. You need to break down any remaining surface tension, and a toothbrush is the ideal tool.
Use a clean (brand new) piece of fine scotchbrite (or scouring pad) and rub over the surface – this will remove most of the finish, don't worry, the next time round applies a much thicker coat.
Repeat the bluing and scotchbrite steps 2-3 times, until you're happy.
Submerge in wd-40 (or alternate water displacing, pretty much just paraffin, equivalent).
Allow to dry again, and apply a light oil – I like sewing machine oil for anything I have to handle regularly.
The result you get will vary a little with both the surface finish you start with and the type of steel. You are literally applying a thin coat of rust, so anything that inhibits rust will reduce the effectiveness of the bluing. Mild steel works best. Tool steels are more difficult. Highly polished surfaces also resist bluing. You want a good consistent surface finish, but not a high polish.