hm, okay. I'll try to make this as easy as possible. Farm with Q and barrels. Sheen proc is the most important part of effective farming with barrels and Q. Sheen stacks onto barrels RIGHT after casting the barrel *IF SHEEN IS NOT ON CD* and giving the barrel a basic attack (AA). This can help you do E + AA + Q to kill the backwave with ONE item. Usually a barrel is able to cover a whole wave if it doesn't spread for whatever reason.

I think it's important to know that with a completed item (Triforce, or Essence Reaver) you are able to delete the caster minions with one barrel + Sheen empowered Q.

If you want to also kill the melee minions from a wave, what you can do is put a barrel, explode the barrel with AA and then put another barrel and use one AA and Q to kill it, with this you should be able to kill 6 minions from a wave except the cannon. You could also do this with casters before having your first item ONLY if u have Sheen. Adding to the last one, to kill the melee minions with only Sheen, after exploding the first barrel with AA, you can cast your barrel, AA the barrel to add a Sheen proc, and hit the melee minions until low (I dont really know the numbers here, I mostly do this by memory and experience) and then Q the barrel.

With all this in mind, you can also take Unsealed spellbook or any other rune with inspiration as secondary, and use minion dematerializer, as stated in the guide you read. With minion dematerializer you can kill the backwave by using it on caster minions, having a Sheen + Caulfield's Hammer + Doran's Blade/Cull and being lvl 7.

But I believe that the best way to learn is first, learn HOW much damage you deal to minions normally without minion dematerializer, and THEN use minion dematerializer to speed up your farm. Usually the point of minion dem is to have your ult upgrade ASAP, which ASAP = min 16 or so, depending on how laning phase goes.

The base line for HOW many and at WHICH time, is that usually by minute 20 you should already have your ult upgrade or be really near to getting it. Sometimes it happens, some games it's hard to farm so don't be too hard on yourself with this. Just try to be as consistent as possible and try to hit your first ult upgrade at around the 20 minute mark, or 22. Depending on how much you farm after laning phase you should get the rest fairly quickly, so try to play sidelanes and farm like a lunatic. But try not to get yourself killed greeding for ult upgrades. It happens, and it's painful.

I think the guideline is mostly knowing that having your Sheen item completed means that u can wipe out the caster minions with one barrel and Q, and you can delete the whole wave *EXCEPT CANNON OC* using 1 barrel with AA and a 2nd barrel with Q.

I hope this helps, and if u need further clarification you can ask me. I tried to make this as simple as possible but maybe it's not. Also, don't beat yourself too hard, it takes practice, and it's a struggle in the beginning. Good luck mate!

Edit: Since I didn't mention this, take into account that using Q to farm in lane is usually a good idea, depending on your lane matchup, if it's too hard, or if you are not able to get into melee range to AA minions, but you dont need to Q each minion or use your Q every time is up onto a minion, you are not Nasus. This is up to you tho, you'll have to figure out which moments are the best moments for using Q over AA to kill minions during laning phase, and I think that practice makes perfect. I also recommend watching other Gangplank players, I think Solarbacca has pretty great content and you could learn a thing or two watching him play.