Make sure you have plenty of reference material. Most important is a scale drawing to work from. And photos - lots and lots of photos.
There are screeds and screeds if you start trawling t'Interweb where lots of lovely people have been sharing.
If you have any relevant photos sitting in old shoe boxes and the like, I encourage you to also share in the interests of recording history - and they don't have to be super sharp calendar shots! Some of my favourites have been made using Kodak Brownies.
Here's some pages I've discovered:
site - includes many shots from the 1980s.
and Facebook groups
NZR 1:76 Scale Modelling - the genre and
scale for me
The New Zealand Railway and Locomotive Society wagon sets are great. They are 1/64th or thereabouts so I scan whatever I need on the PC and print off in a reduced scale so measurements match 4mm to the foot. i.e. a 10ft wheelbase measures 40mm.
The New Zealand Model Railway Guild also have drawings for sale and I have the excellent early Uc tanker variant ones for reference.
Other excellent reference material are the Fred Lee Rolling Stock books, also available from the New Zealand Railway and Locomotive Society, or larger libraries may have copies.
For more modern-era you may need to track down current KiwiRail drawings - check out the NZ120.org site for a good start.
Another place to try is KiwiRail itself, one Carl dot Vercoelen at kiwirail dot co dot nz. They will send you a list of the plans they have for what ever it is you want then you can pick out the relative BP#, main problem is they only take cheques or cash - $NZ20 for the first plan, $NZ10 each for the rest.
Internet and magazine articles
A real find for me was the Modelling Victorian Railways site where, there are many PDFs on scratchbuilding wagons amongst which, and at the top of the page, is an EXCELLENT pdf by David Baillie on how to very easily make bogie underframes. In reverence I call it the "Ozzie method".
These could be a good start if you were modelling modern era. Suitably modified, i.e. use your own scale measurements, they make the basis for many wagons. From experience, do follow their suggestion of fitting transoms for a more solid stable construction. They are so easy you'll be, like me, tempted to churn out more than you could need for a long while!
Yet another find for me was an EXCELLENT article in Railroad Model Craftsmen magazine July 2009 (most larger libraries should have a copy of this mag) entitled "Essential Freight Cars: 45, AC&F Type 11 tank cars" by Ted Culotta on (mostly) scratch building and kit bashing tanker cars. I've adapted this to take on scratch building some old and later NZR tankers and am very pleased with result so far.
Tip - Watch out for articles like these as they contain very helpful information even when the article is not exacly the subject you are looking for. Usually they contain info on techniques more than anything. Doesn't matter what scale or theme you are in to.
The New Zealand Model Railway Guild also publish "The New Zealand Model Railway Journal" - IMHO the greatest model mag ever - and have a back-issues service and catalogue that reference almost anything NZR.
Lance Mindheim - a custom layout builder and designer's very informative blog. Many tips and how-tos.