A proper installation will enable you to have total control on the exact pressure you'd like on your tyres and avoid any air leakage.
Most of us have gone at some point through the pain of experiencing air leakage when installing valve extenders, not able to put the exact pressure we want on our tyres (as the air going in partially leaks and the pressure meter can't provide a consistent pressure reading) or simply watching them deflate as we inflate them, not knowing how to solve the problem, and ending up either putting an inaccurate amount of pressure on them (and praying they don't slowly deflate) or taking our wheels to our closest bike shop.
Properly installing valve extenders is something you can perfectly do yourself.
Here is how.
Initially, depending on the depth of your rims, you'll have a couple of options:
Using a tube with a valve that is long enough and doesn't need a valve extender. When possible, avoid using valve extenders. This is usually possible for up to 40-50mm rim depths.
If the depth of your rim requires a valve extender, or you already have tubes with shorter valves and don't want to get new ones (cost, convenience, etc.), then you'll need to install a valve extender.
If you're at #2, then you (also) have a couple of options;
Use a tube with a removable valve core -this requires a specific valve extender. This is easier to install and inflate/deflate. You simply remove the valve core, install the extender, and install the valve core in the extender. Both the extender and the valve core should be installed by adding a little* Teflon/Silicon tape to the threads on the valve and valve core.
Use a tube with a non-removable valve core (most tubes come with non-removable valve cores) -this requires the more traditional valve extenders, the ones included with nscarbon wheels. In this case, you'll simply need to unscrew the valve nut and install the extender (same as in 1) applying Teflon/Silicon tape over the valve, then sealing it tight.
Now, #2 above is the situation where we think most of us will find ourselves when needing to install valve extenders. The advantage of using these valve extenders is that they both work with tubes which valve cores can or can't be removed, and so in case of getting a flat, you'll be able to use any tube (the one you have or your friend's). The disadvantage is that to access the nut -when you need to deflate/adjust the pressure of your tyre, you'll need to unscrew the extender (if the valve nut can be reached without the extender), or use a wire, spoke or similar to push the valve nut down with the extender installed, if the valve nut can't be reached when removing the extender.
In any case, once installation is completed, tighten the extender just enough (but not too much so that the tape is damaged and loses its sealing power) so that air doesn't leak and you can control the exact amount of air pressure you'd like on your tyres.
*by a little tape we mean somewhere around 5 layers of teflon/silicon tape.
A video that explains it well from the Art's Cyclery guys;