Here is a simple instructable to 3d print new replacement parts for virtually an object with plastic parts that are hard to obtain or no longer available in this case a 1950's classic car water pump component. Be mindful of your intended parts use, 3d printing may not be suitable for high stress, heat and generally extreme functional uses. You may need to make a two part silicone mold and reproduce the print in a resin of suitable properties.
Collect as much information you can. In this case the original broken part that needed replacing was available broken but still in good enough condition to take measurements.
I can highly recommend getting a set of Digital calipers, normally under $30 from ebay and amazon etc. measure all facets of the part twice before inserting the dimensions into your computer model. Currently I'm using Solidworks but feel free to use google sketch-up Rhino, 3ds max or any other 3D software you can get your hands on. Its easier to check twice now than it is to fix mistakes from incorrect measurements after the fact.
Print out to 1:1 scale drawings of each view of the new part design, be sure to adjust your printer setting so that it is at 100% scale not "scale to fit page" as this can cause a lot of confusion. This step is to save time and money by triple checking you 3D files before getting them 3D printed.
Prepare parts for 3D printing. First find a company whom you will get you parts printed through. Due to my budget I went with Shapeways. To get your parts printed first familiarize yourself with the process you will be using in this case SLS or laser sintering will be used. The only part of my design that may be an issue is the intricate holes that water passes through but after reading about it on the suppliers website it's clear that this process is able to produce such details.
Step 4: Finishing Parts
Parts arrived and fit perfectly. As sls produces a rough finish i coated the part with some epoxy, I just used the 2 part kind in a syringe from my local hardware store.