About dimension : Where do you get the dimension for your models to know if everything is to par .
about preparation of parts: does one fix , trim and bag or detach only what you need
about modification: each is hown or there are rules
scale drawings, photographs, blueprints what ever you can that you deem reliable and accurate. Its your model, do with it as you wish. Each modeler has his or her own preference as to the method they use for parts preparation and assembly. IMO the more you handle a loose part the greater the risk is to damaging or losing it.
Frenchman: I always recommend to newcomers the following Kalmbach Publishing book: “How to build realistic AFVs and Artillery”.
It’s at many hobby shops. A good read there will give you some info on basics and what not. Lots of tips and discussion about tools, etc.