parts comes down to two basics:
You need to make the actual mold to cast from, which is
usually made from rubber / silicone. So you
need those chemicals. The mold allows you to reproduce your
fabricated model part many times.
You need to use resin to cast the actual model
part, using the previous mold you made. Once the mold is
made, the resin is poured into the mold. When cured, it's
taken out of the mold.
takes practice to produce resin parts. You not only have
to master the resin casting, you have to master the mold
making process first as well.
vaccum chambers and pumps draw out the gasses that cause
bubbles - those will or can form on the cast part. The pump
gives you a smoother part, with a lot less chance of air
bubbles and pinholes. Two major things that will ruin
a cast part.
are dozens of videos on youtube showing how it's done. It
can be fun at times because you can reproduce multiple parts
that all look the same, but you can also custom make parts
for your own model project. Which makes your model more
accurate, or just better looking than others who have built
the same kit.
it's something you scratch
built the first time, it's often easier (with less time
and money) to replicate the part in resin.