for infantry I'd suggest avoiding resin, it's too fragile for small, thin models, I'd recommend using casting metal, you'll need as a minimum some high-temperature silicone (gedeo siligum works up to 300 celsius and is cheap and easy to work with) and some suitable casting metal, I've had good results with tiranti number 4 alloy, although this contains lead, you'll need a metal ladle to melt the alloy in too, you can do it on a kitchen stove top if you have an understanding wife
there are many tutorials on making moulds but the basics are to half-embed your piece into plasticene inside a lego mould box, stick a few beads in to act as mould keys and help align the mould halves. Mix up your silicone according to the instructions, de-gas if possible, if not, get a silicone with as long a cure time as possible and pour it in a very thin stream from high up into the mould, wait till fully cured, flip the moulded object, brush the mould with baby oil or vaseline mixed in white spirit to stop the silicone sticking together, then repeat. You'll need to cut an ingate to pour the metal in, and you will need to cut very fine vents to encourage the metal to flow into all the extremeties
melt your metal to a sensible temperature, give your mould halfs a fine dusting with french chalk or talc to lubricate them, put them together and secure with some wooden boards and a g-clamp, pour in the metal, bash the mould with some pliers to encourage it to settle, wait about a minute then remove, the first casts often fail as the mould needs to warm up, repeat a few times and re-talc the mould every 5 times or so
congratulations, you have just cast your first miniature