Fixing code P0171?

I have a P0171 (system running lean) error code, started once a month now comes up almost every day (but I also drive more often now).

I have changed the air filter.
Put additives in fuel (to maybe clean injectors).
Cleaned MAF sensor (it was really dirty, there was been a K&N air filter used in the car before, thinking some oil of that got on).

At idle my short-term fuel trim is -20/-15 and long term is normal around 0 so -5/+5

At load my short-term fuel trim is normal around 0 so -5/+5 and my long term is really bad around +30% (tested on the highway at around 65 m/h)
btw every time the engine light came on was on the highway going 60 or above.

The above results let me believe it is a fuel problem, but please correct me if I'm wrong or if you have other suggestions, I'm no mechanic.

I bought a fuel pressure gauge set to test the pressure, after some research I know how to do it and what to look for, but my main question is where do I connect the gauge to my car, I know I need a fuel pressure line or such, could you please point out where it is located? Maybe with a picture or diagram.

Many thanks!