Hello Chad!
I see you got some nice, quality Polish paper models - I'm sure you'll have a lot of fun with them - and I would be glad to help you whenever you feel like writing - in this thread or a PM.
I even have the first one completed - ulnuckily no photos, and I gave the model away as a gift to a friend.
Now generally the best method of reinforcing parts with cardboard is to cut them out coarse (meaning leaving a few millimeters of unprinted paper around the actual outline of the part), then applying glue - spray type glue works great here. White glue can be used too, but there are some types of white glue that distort the paper part when applied on the whole surface - those should be avoided. Then you glue the part on the cardboard of the right thickness and let the glue dry throughly. When the glue is not fully dry you will notice the reinforced part is somehow "funny" to cut, gummy or spongy. This goes away when everything is dry. Then you cut the part out "fine" - exactly on the line. Tight corners are best done with a sharp knife and not with scissors. The inner corners and the gun cutout on the part you posted are places where a nice break-away type knife will really come in handy. Use that break-away feature often and always have a sharp point at hand - you can gae a much better quality model this way.
Please note that the part you have shown and generally the overwhelming majority of parts that need to be reinforced with cardboard don't need to be folded. The dotted lines in your example are just locations showing where other reinforced parts will connect.
By the way - the inscription in the lower portion of the part tell you to sand the part a little to form an edge ("szlifować" means "to sand" in Polish).
The slots in parts "W-14" and "W-15" are also best done with a sharp knife. Don't worry and cut the base of the slot with a cut a little wider than the slot itself - makes for an easy work and doesn't make the part significantly weaker.
One more thing - the bulkhead somehow almost always have a tendency to be a little too big. Before glueing check them for fit with the outer skins and sand down the edges a little bit to make them fit.
I hope this helps - if you have any more questions, please don't hesitate to ask. good luck with your build and have a nice day
Paweł