Trim is generally situational, there's no perfect trim for all conditions.
To trim an aircraft, first start with elevator trim and decide on one of two things: the climb rate or the speed you want it to achieve when flying (almost) hands-off.
Because airflow (and thus lift) over the wings is affected by your airspeed (your speed relative to the speed of the air outside), changing one also changes the other.
From here on there's some counter-intuitive stuff happening because for most people elevator trim raises or lowers the nose, but it helps to think of it in the following way.
What elevator trim essentially does is tell an airplane to fly a certain airspeed. If it can't, it will either stall (if you tell it to fly too slow) or raise the nose (if you tell it to fly too fast). Conversely, if your speed changes for whatever reason, the aircraft will raise or drop its nose to compensate and keep at the speed trim tells it to fly.
This might result in your aircraft climbing or losing altitude, but the plane doesn't care. All it cares about is keeping to the airspeed dictated by elevator trim.
So, trim nose up (slower) or nose down (faster) until you achieve the speed you want. Climb is then a byproduct of speed and leftover engine thrust: if you increase power the plane will raise the nose to prevent an increase in speed and you will climb, if you decrease power the plane will lower its nose to prevent a speed decrease and you will descent.
It's pretty similar to how we've been told to think during approach and landing, ie "nose up/down governs your speed and power governs your climb/sink rate".
Various scenarios are then possible and with some experience, you will be doing it so naturally that you will know both the limits of your aircraft and what changes to expect in both airspeed and climb rate due to a trim change.
So, once you have finalized your elevator trim for the power setting, speed and attitude you want to attain (eg, a modest climb of 500 feet per minute at cruise power and a speed of 220mph, or a gradual descend at 500 per minute with low power and a speed of 200 mph), you can focus on the rest of your trim surfaces.
The reason you take care of elevators first is that they tie in with power and power affects sideslip. Without going into too much details, that rotating propeller in the front adds both a rolling and a yawing component to how the plane moves through various phenomena (rotating airflow hitting your vertical stabilizer, gyroscopic torque, yaw affecting roll, etc). So for example, if you trimmed your rudder before finalizing your climb/speed/power trim with your elevators, you would have to redo your rudder trim after adjusting power. That's the reason you first adjust elevator and then rudder.
With the rudder things are pretty simple. Just adjust left/right until the sideslip indicator reads zero.
Of course simple doesn't mean trivial. As soon as you trim your rudder properly your aircraft flies straight, nose-on through the air, whereas before it was flying at an angle that exposed part of the fuselage to oncoming air. You were flying with extra drag before, which you now don't have anymore. As you can guess, lack of resistance means an increase in speed and since our elevator trim wants to keep speed steady, the nose will rise and you will climb.
So, a minor adjustment in power and/or elevator trim might be required to bring it all back to level.
Finally, ailerons are the last to trim. It is often the case that once you trim everything else, you will notice that you are rolling off to one side. This is both due to the propeller effects as well as the rudder being deflected slightly (because you trimmed it and it's now sitting off-center) and causing a bit of roll. So, just trim ailerons left or right until you zero that one out.
Once again, this might change your sideslip a bit and require a touch of rudder trim correction.
Overall this is the process. Don't be daunted however, because it takes much longer to describe and read/understand than it takes to practice and apply it. Good trim is an essential skill that few virtual pilots master to a high extent, but the majority are able to learn to a satisfactory level.
Last but not least, depending on your aircraft choice you might not have all 3 axis of movement equipped with trim controls. Some aircraft lack aileron trim (most if not all of the RAF ones), some lack rudder trim (like the 109), etc.
In that case you do one of the following.
-Compensate with actual control inputs: You can't really fly hands-off.
-Stick to the airspeed for which the missing control is pre-trimmed by the manufacturer: Ailerons in RAF aircraft have a bit of built-in trim that corresponds to their nominal cruise speed, flying that airspeed will require no aileron adjustment. Similarly, the 109 rudder was adjustable by the ground crew before take-off and trimmed for a specific airspeed.
-Fly a bit inefficiently: If you want to fly faster than the built-in trim allows on the non-adjustable surfaces and you still don't want to fight the controls all the time, it might be good to compromise. For example, flying a Blenheim with correct rudder trim induces a bit of roll. If you slightly move your rudder trim away from optimal so that you get a bit of sideslip, it will prevent it from rolling. You lose a bit of speed in the process, but you don't have to fight the stick forces all the time.
Hope it helps. Generally speaking, if you have a bit of an idea on how things work it's easier to test it out for yourself and gain experience.
