Rookie question, despite my time in the hobby. I've never assembled multipart minis. Curious how you approached it. Have an idea of what you'd like and find those parts on a sprue? Break off all of the parts and shuffle them around until you think you have the right head/arms/legs/torso/pose/weapons. Do you blutack them first to see how they'll look?
Any lessons learned so I'm not attending the "Invention of the wheel, part one" class on my own
Before getting the tools out, spend some time looking at the parts on the sprue, and checking them against the box art and/or any instructions. Having some familiarity with the parts, and knowing what they are "supposed" to look like is a really helpful starting point! Then again, I am the pedantic sort who always RTFM...
I would also make sure that you consider whether any arms are paired (two handed weapon arms often are, and may be labelled on the sprue some like "21, 22" or "21a, 21b"), and what kinds of poses you are looking to acheive.
Other than that, pick a body and clip it out, deicide what weapons and head you'd like, find and clip those, and then clean the parts up. Spend a little time dry-fitting the pose (some poster tack may be helpful), and then glue them together.
Once I'm done building all the models, I tend to clip all the remaining spare parts off the sprue and put them in a labelled ziplock bag so that I can use them again in future. I bag paired arms separately if it's not obvious how the pairing works, or if there are multiple different but similar paired arms.
General suggestions to keep in mind:
Think about the poses the models' legs and bodies suggest. Try and find arms that would fit these poses, to avoid things which may look odd: like a running pair of legs paired with carefully aiming arms, or a kneeling pose with arms in a close combat position.
Make sure you clean all the mouldlines off properly before putting the model together. Mouldlines on plastics can be heavy, and they can be extremely difficult to reach if you do this after a model is assembled. I personally find that mouldines detract enormously from the look of a finished model (even one with an extremely basic paintjob), but it seems plenty people can be less fussy than me! Not trimming and cleaning the parts thoroughly can also prevent you from getting a good fit of the parts.
If a pose isn't quite right, cutting arms at the wrist and trimming/filling shoulders a little should get you there. This can also be a good way of swapping weapons into different poses etc too.
Positioning the head to suit what the model is doing is critical to achieving nice-looking poses. For example, guns held high across the chest can have a head looking across them, or looking in the approximate direction of the barrel. However, guns being properly aimed need to have the head positioned to look like the model is sighting along the barrel, or they will look odd. I like to put a thin wire pin in the necks, as this helps me get the pose right before I glue it in place, and the thin pin also helps to strengthen any poses which might move whilst you're waiting for the plastic glue to cure (again, with "aiming" head positions, this really helps...).
Try and avoid a "both arms straight out" pose, unless it is specifically relevant to an action you are trying to portray the model engaged in. This avoids the Lego or Playmobile Man look.
If the model has weapons drawn, make sure to trim any weapon handles out of holsters (ditto swords on other plastic figures). In reverse, if a model isn't holding their weapons, make sure any holsters have a handle sticking out of them.
With a fine drill bit, drill a little divot up to 1mm deep into the ends of gun barrels. On plastics, you can "mark" the centrepoint with an awl or such, and then drill with the bit in your fingers - the plastic is soft enough that this works for shallow holes. This will make the guns look much better!
Consider (and this is more of a preference thing), whether you want to leave an arm or weapon off for painting if it makes getting into the model with your paintbrush difficult. Afterwards, a quick scrape of the contact points on the parts with your knife to remove any paint, and then glue into place.
Finally, try to enjoy the process, and don't worry too much if some models look a bit awkward or not quite as you like. Depending on how strongly you feel, you can always cut the arms off and try again later, or not. Afterall, do remember that there are plenty of metal models out there with awkward poses too!
Hopefully that's helpful, although I realise a lot of this is pretty obvious and mostly applies to metal models too.