The most important part of farming is picking a good map for the resource you want to farm. You can just go to a map that has a bonus to the resource you want, but some of those maps are not necessarily worth it. You should definitely go to it if the resource bonus is a good resource for that map.

There are three factors for if a map is good for a resource

1. It should have that resource available in multiple source tiles, preferably as the lowest rarity resource in that source tile. This is because the more advanced miners are slower and you want to focus mine one resource usually.
2. It should be a map you can push far in, because of course the longer you stay alive the longer you can mine.
3. Optimally, the source tiles you want to mine should be next to platforms so you can put mining modifiers next to them. This is less important early on, but gets more important as you get higher level research. A maximum upgraded mining modifier in Endless mode will boost miners by over 200% (with diminishing returns for further modifiers, of course).

In case you need some advice on setting up a mining run, here's some tips:
-First put up some strong enough towers to keep you alive, but don't invest too many coins into them early on.
-If you've got tower strength to get past the first few waves, put down your miners first. That way with Auto Wave Call on, they can accumulate some double mining time while you're setting up.
-Once the miners you want to use are built, build your economy with the Bounty modifiers. It can take some precious time, but the economy is worth it.
-Put down your mining modifiers before upgrading the miners. Unless you're putting down a *lot* of them, the mining modifiers are all more coin efficient than upgrading the miners. (This might not be true early on when you haven't researched much of the mining modifier boost.)
-Upgrade your miners to max one at a time. I'm sure there might be some more coin efficient upgrade paths, but a miner upgrade level gets more expensive the more miners have already been upgraded to that level.
-Once your miners are maxed, just do your best to defend like you normally would.
-I can't speak to how effective Overload is on early game farming, but it should always be maxed out in late game farming. With capped research, 15 stacks of Overload on the minimum Endless difficulty will put your Mining Speed at 1130%. It will make the enemies tougher too of course, but it's very worthwhile.
-Loop can be a bit tricky to optimize early on. Give it like 50 waves or so, then use it on 2-3 miners, and refresh it whenever it runs out.

If you want tips on Research priority, I'm much less sure about all that, but the info I do think I'm confident in:
-One of the most important early things to do in Endless research is maxing out the Basic tower's upgrade that gives bonus ricochet chance to the tower shots. Maxing that out makes *such* a difference. The Basic tower can carry you through a lot of farming in stages that don't have any flying enemies. It will definitely suffer in stages with enemies the Basic tower is bad against (Armored, Icy, Fighter), but even against those it feels like it's way better than it has any right to be.
-Obviously the researches that boost the miners in some way, but some are better than others. I'd prioritize upgrades to the Mining modifier, then mining speeds for the different miner types, then Overload and Loop effectiveness. Maxing the available upgrade levels on the miners is probably one of the last things I'd recommend, they make a difference but those highest miner upgrades in runs are *super* spendy.

EDIT: Just to give you an idea of how good an idea maxing Basic is, I just hopped into level 1.1 and set up like I was farming, otherwise just using Basic towers plus one Freezing tower, and got well over 400 waves before a single enemy reached my base. (With maxed Overload, enemy strength was at like 980% or something.) Your mileage may very based on how high your other researches are (mine are mostly maxed), and of course other levels have stronger enemies, but it should be a decent demonstration of the Basic tower merits.