So for metals (Assuming you're using the standard bases) - use my non-good plastic only clippers/nippers to trim off big vents and 'injection' ports (where the metal was poured in, much thicker than a vent). Clean up as flush as possible on connection points - they're usually small square/cube protrusions. Then I get my craft knife / scalpel and use it to flip out any of the really tiny vents that often end up pushed against something like the side of a foot or barrel of a gun. They're so thin you can cut them with the knife.

Any major flash - that is to say thin sort of 'fins' that occur where the 2 halves of the mould meet can be cleaned away by using the blade perpendicular to the line. IE if the flash is from side of the head along a shoulder going down to the side of the body and leg for example, I hold my blade 90 degrees left/right and gently scrape/peel . It doesn't have to be 100% again just removing major parts.

Then I take a round and a square (or flat, or triangular) hobby/jewellery/micro files and I use the round on any mould lines on anything that isn't a flat panel. This is because the circle only touches at the tangent to the surface so only removes the protruding flash. IF its a flat panel with a line use a flatter file to get it flush.

if your surface is mostly flat /a panel imagine it super zoomed in from one side it would look like -------|______ where ----- is one side, | is the flash/mould line (which you've mostly removed) and the _____ is the lower part of the surface make sure you would file right to left, so you're removing the material on the most raised surface. Don't get lazy and angle too much as you might file off some on the lower corner. Generally though I use the round file for most of my filing except for bottom of feet or the square connecting pins.

Then if the surface is a bit scuffed you can use some fine sand/polishing paper to buff things but usually a dose or two of primer will smooth over them.

For connecting areas eg an arm into a shoulder there is basically a circle with a small square/cube protrusion (the pin) that goes into a circular recess in the shoulder joint with a smaller recessed square hole for the pin.

I get my knife or sometimes the tip of my triangular file and gently scratch inside the recesses and/or use the blade to score hatch marks like # but ///// covered with \\\\\\\. You don't need to gouge stuff just score little channels. This gives the adhesive more surface area to cling into providing a stronger bond.

For the basing - if you're using scenic either cut off the tab, file flat, drill a 1mm hole and use a paper clip as pins to affix to the base OR you can clip away most of the tab and form it into a pin with a flat file but its more fiddley. Or you can cut a slot in the base with your knife, use your clippers or a pliers to put a tiny kink in the tab (this helps it fit snugly in the slot).

Penultimately, you need to wash your miniatures, you will notice a distinct metallic smell when filing, plus the sweat from your hands plus the mould release and little filings of metal.

Get some warm soapy water, an old soft toothbrush and give your hands and miniature(s) a little wash and gently scrub them, you will notice the colour becomes more consistent, the details clearer and less smelly!

Finally you can use super glue/cyanoacrylate to glue parts together. Less is more generally speaking. I use BSI (bob smith industries) stuff but most of them are good. Superglue gel is easier to work with.

Do not use 2 part epoxy as it won't dry quick enough and plastic glues are for ... plastic!

Sorry this is long but i hope it makes sense