There's not really a step by step as it honestly doesn't really require one
I can put some thoughts down for you on how you could prioritize going about this though.
The circuit of the amp will remain untouched - so you don't need a ton of knowhow to do any of the mods. All you're doing is taking the components that are in there and replacing them with better components, same (or similar) value, same place in the circuit, just better 'stuff'.
For Noise reduction - this would be where I would start
1. Copper Flux Band mod - this is your greatest noise reduction and it's relatively simple to perform, albeit a bit tedious
2. Regulated Heater Mod - I've heard this on a 339i without the copper band mod and they work about as well as each other, I plan to do both on my amp to get noise to an absolute minimum
3. Elevated heater mod - this worked, but to much less of a degree than the other two (I've employed this on mine amp regardless)
4. Twisting the heater wires - this is super easy and should be done regardless of any of the above.
5. Reroute signal wires to Y topology - currently these run around the case of the amp and can pick up noise, I rerouted these through the middle of the 820uF electrolytics
For 'Better' Sound
1. Replace ps electrolytics - these are the big ones in the back, i think 820uF originally i stepped them up to 1000uF with good results
2. Secondary ps electrolytics - there are 4 of these at 220uF (i think?). Those physically closer to the big caps in step one aren't in the signal path and I replaced with Mundorf. The ones that follow these are in the signal path and I replaced with Audio Note
3. Resistors - I used vintage carbon for the ones across the tube sockets and Kiwame elsewhere (these give a lusher sound IMO). I also used Audio Note silver for the 100ohm I believe on the signal output.
4. Coupling Caps - These are the 150uF ones under the middle transformer shroud cover. I took this off and upsized to 230uF, more impact, more bass - wasn't exactly cheap or easy though. Problem is there isn't a lot in this size that will work that aren't electrolytics - ideally you want a film cap. Mundorf has some but I found those to be very neutral and unexciting sounding.
5. Volume Pots - why do all this and not replace them? I took the cheap ones out and replaced with TKD 100k ohm (non stepped, why have stepped for two independent channels....)
Tubes
This had a significant impact on the sound as I'm sure you're aware.
1. Power tubes I went with Western Electric 421a
2. Input tubes I went with L63 GEC
a. Yes these are 6J5 tubes but as long as you rewire the tube socket or build an adapter these will work without additional circuit modification. Some have seen success with changing the 200ohm resistor to 100ohm and the 33k ohm to 24k ohm with these tubes but I've done neither and it works just fine (I may put a switch in place of the LEDs to allow me to move from 33k to 24k at will, TBD though). They are a bit less 'linear' sounding as 2nd order harmonics are increased but you get more of a tubey character to the amp which I really like.
I think that's about it? Probably took me close to 20 hours in total to get all this done though I didn't do it all at once. You're literally taking out a part that's in there originally and just replacing it with a newer 'better' one. It's very simple and you don't need much experience to get it done, you just need dedicated time
If you get stuck or have more questions I'm just a PM away!
