I will go to a wet territory (Jotunheimen) next week and would like to prepare my full leather shoes for that. I have read a lot of articles about shoe care and they are either about impregnation (spray) or waxing but what to do first? Could anyone tell me in what order I have to treat my shoes so it does not harm the leather and I get the best water resistancy effect?
Full leather shoes are easy to take care of in a way: they'll perform exactly accordingly to how you treat them.
First off: if the shoes are new, don't to anything. They'll already be treated/impregnated and ready to use (apart from breaking them in). You'll likely not gain anything by applying additional impregnation.
Treatments I have experience with:
In my opinion natural bees wax is your best option. Makes the leather 100% waterproof, but keeps it breathable, so less sweatiness for your feet. Works best if you warm your shoes up (e.g. using a hair drier), then apply the wax, then warm some more until the leather soaks up the hot wax.
Is quite similar to wax, but from my experience (and what I heard) it impairs breathability of your shoe a bit, so you're more likely to sweat.
Imho this is good for short duration uses, but needs to be repeated frequently. Also if you do hikes that really tax your shoes (i.e. lots of water, walking through high grass, underbrush, rocks/sand, ...) it is my experience that the impregnation will rub off/disappear quite quickly. So for a hike of a week or more, I'll stick with grease/wax.
My personal favorite is natural bees wax. Makes my full leather boots 100% water proof, keeps them breathable and I don't have to repeat the procedure very often... One waxing before a 2 week hike is enough for example (I repeat it maybe once a year).
I have no experience combining the above techniques, and don't think it's worth trying. Find something that works for you and stick with it.
Note: if you're using wax or grease, don't treat your shoes more often than necessary. The treatment (especially if using the trick with the hair drier) will also make the leather more supple. This can be a good thing in the beginning, but if done excessively you can lose some of the stability your shoe gives you (e.g. around the ankles).
I treat my leather boots with neatsfoot oil, and that generally is plenty to protect the leather and keep it supple in mud and wet. If it is snow with a lot of salt and things around, I might add something waxy like Snoseal or mink oil. Overall, neatsfoot oil is cheap and works wonderfully. Generally it is all I need spending a lot of time outdoors in Upstate NY.