There's are some tricks you should know about the maxiball. They generally work best with a felt wad under them, this is true of a non-hollow base great plains type tapered bullet too. I also find maxiball shoot best with really strong loads. Some rifles shoot them great slower, but more often than not I find that 100-120 gr powder range the best. I had better results lubing only the bottom groove of them, however, powder fouling with any conical is worse than a patched ball. You definitely need to have a good barrel swabbing routine.
Lyman makes a muzzleloading book that is worthwhile to own. I don't think it's worth buying a casting specific manual (unless you want to start casting for handguns/rifles), as a lot of it can be outdated information from when NRA 50-50 lube and linotype lead roamed the earth. There are some books specifically about rifles that can be good, but for muzzleloaders I wouldn't bother. Most of what you need to know can be found online, just be aware that there is more than one way of doing this. Casting bullets is manufacturing, and there's nothing wrong with going super cheap, or going all out. Some people cast bullets in a cast iron bowl on a propane stove and eyeball everything. It works fine, but the final product can vary a lot. On the other end of the spectrum some are super anal about the casting and alloy, temperature, sorting, and every last detail. Those are often rifle shooters getting cast bullets to perform like jacketed bullets.
You really can get good results casting with just a cast pot and a propane stove, but you really should get a thermometer for it. Without knowing your lead temp it can be really hard to get consistent results. For pure lead I start at 800F, which is hotter than you will often see recommended. I often go as high as 830F as needed. A lot can depend on the mold you are using. An iron mold like you bought generally works better with a lower pot temp, because the mold will hold heat better. It's really the mold temp that matters for good castings, not the lead in the pot. I would start at about 775f for that Lyman great plains mold. The Lee 4-20 pots work really well too. This is one area you will find debate on bottom pour vs ladle pour casting. One of the factors to getting good castings is fill rate. There is nothing inherently wrong with a bottom pour, but most pots are a little on the slow side for bigger bullets. They are generally ideal for 100-250 grain bullets, which is what most people cast. Some might pour faster or slower. I own two 4-20 bottom pour pots, and one 4-20 ladle casting pot. Both my bottom pour pots are too slow for 300+ grain bullets in my opinion. You can make ok bullets with them by playing with pot temp and casting cadence, but it isn't what I like to do. I actually use one of the bottom pour pots with a ladle, I just turned the spout so it can't be used. For muzzleloader bullets I would definitely opt for a ladle. You can make really good quality bullets with a ladle. You can get them in different spout sizes, but the Lyman casting ladle, which I recommend, is pretty large. It works good for 200+ grain bullets. I've gone as large as 1000 gr shotgun slugs with it, and it still does ok there.
For muzzleloaders, you don't need, and probably don't want alloys. Pure lead is the best choice most of the time for round balls or conical. The softer lead is easier to engrave in rifling, and bumps up to size better. The only time I would opt for a harder alloy would be if I were shooting a pistol bullet in a sabot. As I said earlier, a slightly addition of tin is acceptable. Tin lowers the surface tension of lead which can make casting easier. It does harden the alloy slightly, but not by a ton. 20:1 is as hard as I would try, but 40:1 is more what I would be going for. When I say "pure lead", I don't mean it has to be truly certified pure. A common source of soft lead is pluming lead from pipes. It has some impurities, but it is like 99% lead. Plumbing lead makes great muzzleloading bullets. Roof flashing lead is another source of soft lead. It can be found as scraps from roofers. There are other sources of soft lead, but those are the big two right now. Alternatively you can buy lead and lead alloys. You can find it for sale online from fellow bullet casters. You can also buy it new from places like Rotometals.
You will probably mess up bad your first time casting, and that's ok. There is an art to this when we are working with handheld molds and pots of lead like this. One of the biggest variables to casting is your cadence. What makes good bullets is doing things in a way that keeps your mold at the correct temperature. You can raise or lower the pot temp to have some effect. The most direct effect you have is the rate that you cast. If the mold is cold, cast faster and vice versa. If the mold is too hot, it will take a long time for the bullet to solidify. This will result in issues such as "finning" (which can have multiple causes), smeared bullet bases, and even shrunken/odd shaped bullets. You will also find reference to "frosted" bullets, however, this is a product of antimony, which you do not want in your alloy for muzzleloader bullets. Pure lead, and lead-tin alloys do not frost. Too cool of a mold will usually result in one or both wrinkly/ugly bullets and rounded bullet bases. They will also usually be undersized. This takes time to develop the experience for keeping your mold temp in the right range. I don't buy into the set time of say 6 seconds for a sprue to freeze as an ideal temp. Some molds I find work better with a quicker freeze, some work better longer. Along with that, it is a bit of an art knowing when to cut the sprue. With soft lead it's less of an issue as long as your bullet base is sharp without finning. You probably wont get divots from cutting the sprue as easily as an alloy with antimoy.
Once you get decent cast bullets, the next thing to do is sort them. Even the best casters get some reject rate, and often beginners are over 50% rejects. What constitutes a reject can vary widely based on use. For a pistol shooting ringing steel at under 25 yards pretty much anything that looks useable is good to go. For muzzleloaders, since it is relatively low volume, I do like to be more selective. The most important thing is that all your bullet driving bands are filled out well. If any appear rounded, or you get any wrinkles in the side, dump that bullet. If you want to take it a step farther, you can weight sort. Where I differ from some is I only weight sort to the heaviest bullets. Weight sorting into groups is not a good plan in my opinion. The reason those bullets are lighter is because they have defects, sometimes visible, sometimes not. You do have to be careful you aren't getting over heavy bullet due to an issue such as a mold that was not closed all the way. I like to measure my test bullets with a calipers to make sure they aren't oversized, which would cause them to weight more than they should. Lets take your great plains bullet as an example. Lets assume you take 15 from the pile and weight them. Lets say they weigh from 383.1 grains up to 395.6 grains, and there are at least 4-5 that are at or above 395 grains. I would then measure a bunch on the top driving band and make sure non are oversized. If all checks out, I would then sort to that weight. There is no set number on what amount of weight variance is acceptable. Starting out I would say anything within 2 grains is fine. For that I mean I would dump any bullet that weighed less than 393.6 grains. Any bullet heavier than that would be a keeper. If I got some that were 396.8 or heaver, I would set them aside and reevaluate them.
I wont go into lubes much, there's tons of info out there, and not really a wrong way to do it. For my muzzleloader bullets like that, I like a 50/50 mix of beeswax and olive oil, mixing by weight. I do what is call pan lubing for my maxiballs.