So here's an example of how it goes. Each step is easy, it just eats up time.

First, I use a cutting board to make sure everything is rectangular. The steel edged ruler USUALLY keeps edges straight.
  

In this case I used the first set of seat I cut out as a template and just cut the same lines into a second piece of plastic.

Easy. I can use the same trick in other places. For example, the rear wall of the troop bay. Here I've used a pencil to trace out the door frame.

This ladder is made using the ruler again. Then I simply cut out each section - note this is all much easier with 0.5mm or 0.25mm plasticard, and NOT the thicker 1mm stuff.

I then use the same cutouts for the steps - recycling makes things easier!

I've been using plastic rivets I found on HLJ and ebay, but you can easily use plastic rod instead (or even plasticard cut fine if you don't mind a square cross-section). Just cut the rod finely as shown here, and glue in place.

Here is some of the original concept art from a decade ago that led to this tank. I always had plasma in mind for sponson weapons as existing Astartes vehicles don't really use it much. I sourced part from Ebay in the end as I wasn't keen to try scratch-building these (though I now think it would've been easier than I'd thought).