Im no expert but this is my experience.
In hindsight, a good way to start would be to investigate how and where the sensor for your gauge is going.
For example, my water temp gauge sensor cannot be fitted easily where I want it to because the thread is too close to the thread of the hole I want to put it in. Ill have to take off the manifold so i can drill and tap and get all the metal shavings out.
Knowing the thread size of your fittings and sourcing adaptors before you decide on a gauge would be less of a headache.
Oil
These are another option for oil, much easier to get to the filter area than under the manifold.
Water, im going to put an in line adaptor in the top radiator hose.
They are 38mm or 1 1/2' for the R32 and are available at autobarn, supercheap, etc
Fuel
Boost
So, get your gauge/s.
And tools and materials.
Make a wiring plan because it can get confusing especially if its already a mess(like mine).
Route your wires, do them all at the same time, its easy to add an extra one or two if you need them later.
Once you get a bunch of wires in there it gets harder to push more though.
Know where you are drilling to avoid disaster, i have a good dash in the shed to check where to drill, and the one in the car can suffer my mistakes.
Solder, heatshrink and good plugs are good to use to get it sorted
Mounting in the vent, need a slot for wires.
Perspex for the console, sprayed black on the back.
A bit messy, these wires need to be sorted.
All done