I want a recoloured version of the suspension bridge available in CAW. I've never tried to do this before, and I really want to try to figure out how to do it myself... but can't seem to find a tutorial that starts at the beginning. I can find several sources that explain how to install such custom content, but not how to go about making it for myself.

Hi, I personally start with cloning the object which I want to recolor at S3OC, but clearing the box "renumber internally", so it will be a replacement. Then open the package with S3PE, export the _IMG resource, keeping the same name. After editing it with Photoshop, save it as DXT5 ARGB 8 bpp interpolated alpha, generate mip maps checked. You need dds plugins for photoshop.
Open your world file with S3PE, import the dds file. Save your world and open it with CAW.
Make sure you back up your world file.
Hope this helps!

Where do I find the object, in order to clone it?

After opening S3OC, select cloning from the top menu, then normal objects. (BridgeSuspension)

I have always used the S3PE to get the dds files for recoloring straight from the game files. In this case it would be "The Sims 3 Late Night Expansion Pack -> Game Data -> Shared -> Packages -> FullBuild_p06.package". Open the 'FullBuild_p06.package' file with S3PE, find the needed suspension bridge IMG files and then choose: Resource -> Export -> To file.

Now you can recolor those IMG files as you please and when they are ready, you can simply use S3PE to import them into the world you're are working on. Just open your CAW world file with S3PE and choose: Resource -> Import -> From file. It's quite easy and quick to do.