I did a fair bit of resin casting in the 1990s. I found there were three key factors to success:
1) Learn your mold-making and casting processes well.
I used paint-on latex for mold-making. It was a bit tedious.
It took many layers to build up a robust mold, and it took hours for each layer to dry. I learned it was important to make the mold pretty thick, and to build-up the right amount of border around the master object (a flat area of latex about 1 inch around whatever I was building a mold on).
Then, in the casting, it was important to suspend the molds securely. I used small cardboard boxes (like pizza boxes), with cut-outs to various rectangular shapes. The mold would be suspended through the cut-out that most closely fit it, and the flat border had to be taped down very securely. Only then could I move on to the actual mixing and pouring of the resin.
I used a dual compound resin. It was very (VERY!) important to measure the compounds correctly, and stir thoroughly. Few things are more frustrating than taking all the time only to find that the resin WON'T CURE in the mold because you mixed it wrong! You not only lose the casting, but you probably LOSE THE MOLD! And those molds take MANY hours to create!
I had a friend who used to frequent this forum who did home molding with a silicone mold-making compound. It was a LOT faster and easier for him to make the molds. He poured the compound into a small container (usually a dixie cup), pressed the master into the compound, and in about 15 minutes the mold was dry enough for him to pull the master out! Wow. Also, his molds were so heavy and blocky that he didn't need to suspend or brace them in any way. Just put it on the table and pour the resin in. Wow, much more convenient! But ... his molds only survived making about 2 or 3 castings. Mine lasted for dozens of castings.
2) It takes TIME!
It is not a fast process. It can be a lot of fun, but don't convince yourself that you are saving lots of money, because, well, you are probably worth more per hour than you are saving.
Still, it's kind of cool to be able to reproduce stuff when you want to. So if you're into the fun of it, GO FOR IT!
3) Learn to make scratch-built masters.
Here's the thing. There is no reason to cast something you can buy commercially. Firstly it's neither legal nor ethical. But even if you are a willing pirate, at this scale with resin you'll never be able to cast at the details needed to pirate fine models (like tanks ... not even brand-x tanks, much less GHQ!). Mostly what you will be able to cast are terrain features. And most terrain features are not that expensive to buy. And even if you really have more time than money, you can save even more money by creating your own masters than buying commercial models to pirate.
But ... you have to get good enough to make it worthwhile duplicating whatever it is you can scratch-build!
I did some artillery positions...
Here you see one of my artillery positions in use in a game, occupied by some of my GHQ Italian howitzers. I thought they were pretty good at the time. But now I'm less impressed. Still, they are down-right useful.
I cast about 5 of these. Some were painted and flocked, as you see here. Others I wound up giving away to gaming buddies in their raw un-painted state. The thing is, who really needs 5 artillery positions? But everyone can use 1 or 2! So they seemed to be valuable give-aways.
I've built some new artillery firing positions in the past couple of years. I think they look much better. But I don't need so many that it would be worth my time to re-learn all the tricks and buy all the supplies for molding and casting them, so I just painted and flocked the masters and will use them when the time comes.
More useful than the artillery positions have been my castings of scratch-build buildings.
Most of the houses in this game pic are my home-castings. The church and one house with a red brick chimney are commercially bought. But if you look you can see several of my scratch-mastered castings.
There is a yellow house on the left, which is identical in shape to two white-sided houses near the intersection (one with a gray roof, one with a brown roof). These three are cast from the same scratch-built master. Also there is a larger white house in the foreground, which is identical to a brown house in the hedged-farmyard in the back. Again, both cast from my scratch-built master. Along the road that cuts to the river on the right there are another two of my cast houses ... one dark brown with white trim, the other a kind of dull green with a brown roof. And on the road coming down to the foreground there is a T-shaped house that is one of my scratch-mastered. I have multiple castings of all three of those styles too, though not on this gameboard.
My masters were all done with balsa. I bought two 36" long rods, one with a square cross-section, one with a triangular cross section. The masters for all of these houses were made from various cuts from those two pieces of balsa.
Many of the others are also my scratch-builds. All of the thatched roof huts you see are scratch-built, but I never bothered to cast these as they were so quick to scratch-build. I got a third rod of balsa which was flat on one side and rounded on the other. I just glued a piece cut from that rod on top of a piece cut from the square rod, and then a few minutes with a knife blade to do the thatching/scratching, and they were ready to paint.
So .. in my view the keys to casting are: learn your tools, commit the time to it, and build something worth casting!
Hope that helps.