In principle, aerial refueling is "just" flying in close formation. Doesn't sound too bad, right? Well, it's not easy!
Before you make your first attempt at "putting the thing into the thing" I recommend you practise formation flying. You can do this online with a friend (which I would recommend) or you can do it in the editor.
Start of with a wide formation and as you get more comfortable you can gradually fly closer and closer to the other aircraft until it becomes a routine thing to do.

Headtracking like TrackIR or, if you are good at DIY stuff, Freetrack will make formation flying a lot easier. TrackIR is expensive but I can whole heartedly recommend it to anyone who loves flightsims. In my opinion it is more important than an expensive HOTAS.

• Staying in close formation requires small amounts of constant stickmovement and tiny adjustments on your throttle.
  To make things a little bit easier set a curve on your stick (In my experience 15-25 works really well for beginners). You can play around with it to find a setting that fits.
  More experienced users with precise sticks like the TM T.16000M or TM HOTAS Warthog may prefer to fly without any curves as it gives you an even response throughout the movement range of your stick. In the end it is up to personal preference.
  If you have a HOTAS with two throttles like the Thrustmaster HOTAS Warthog it helps a lot to make the adjustments just with one throttle at a time. Walk the throttle as pilots say.
  
  
• One of the biggest problems for new pilots is to get used to the fact that you always have to look at your lead when you are flying in formation. This is important. Lose sight of him just for a short moment and you are in danger of breaking formation or worse, crashing into your lead.
  So, do not look at your instruments! If you feel the need to check something in the cockpit quickly glance at it and instandly put your eyes back on the leading aircraft. The HUD is absolutely off limits as long as you are in formation so it may help to turn it off completely for practice.
  
  
• Trim your aircraft prior to contact. Seriously, just trim it out as best as you can! The better it is trimmed, the easier it is to stay in formation. I like to trim slightly nose heavy as I feel more comfortable with slight back pressure on the stick.
  
  
• Relax. Get in a comfortable position, do not clamp your hand around the stick. Only make very small adjustments with your fingers.
  
  
• When you're in formation with the tanker don't chase the boom. Concentrate on staying in formation. The operator will take care of the boom.

If you are familiar with TACAN in the A-10C you can skip this part.

Once you feel comfortable in close formation it is time to find the tanker. Each tanker has its own TACAN and radio frequency. The frequencies for each tanker can usually be found in the mission briefing. If the mission author did not change anything it will be TACAN 01X and 150MHz VHF AM.

TACAN

First you need to set up your Tactical Air Navigation System, simply referred to as TACAN, located on the right dash.
Dial in the frequency as required and set the rotary switch to A/AT/R as it is shown in the picture. Once you have done this correctly you should hear an audio signal. I would recommend to turn the volume down since the constant beeping is pretty annoying.

Nav Panel

Now that this is done, go to the navigation panel located at the center front dash and activate TACAN.
Once activated you will see a green triangle on the button. On the HSI (Horizontal Situation Indicator) the needle with the number '1' now points in the direction of the tanker and you will see the distance in nautical miles displayed on the top left side of the HSI.
_____________

Contact the tanker

Now it is time to set the frequency on the VHF AM Radio and contact the tanker via the radio menu.
Presuming your callsign is "Enfield 1-1", the tanker's is "Texaco" and he is flying at 15000ft he will now either answer with "Enfield 1-1, Texaco. Proceed to pre-contact at 15000" or, if someone is already behind the tanker, "Enfield 1-1, Texaco. Chicks in tow, one mile trail, at 14000" (I hope I got that right^^).

Simply climb or descend to the given altitude and make an intercept.

Once you are a few hundred feet behind the tanker open the Air Refuel Control Lever (RCVR) on the Fuel System Control Panel (big grey lever, left dash) and turn on the exterior lighting if necessary (dial to the left of the RCVR).
As soon as the door is open the READY status light will appear on the Canopy Rail Indicator (to the right of the HUD).
Slowly close in on the tanker and contact him again via the radio menu. If you are close enough he will answer your "Ready pre-contact" with "Cleared contact".












Fuel System Control Panel



Once you are cleared for contact slowly crawl up behind the tanker. It is important that you always look at the tanker. Do not get distracted by the boom - remember that you are flying in formation with another aircraft and not the boom.
When you get close the boom will move up and if you are fixated on it you will most likely make stick movement that will break the formation and you can start again (I am speaking from experience here^^).
Continue to close in very slowly and move up a little so that the end of the boom is right in front of you. As soon as you are close enough the boom operator will stick the boom in. Now the LATCHED status light appears on the Canopy Rail Indicator and you will get the message "Contact" and shortly after "You are taking fuel".
If you lose contact simply press the Nosewheel Steering (NWS) button and try again. Don't worry if it takes several attempts to make contact. It took me months of practice to become good at this.

This is how it looks like when I am latched. I marked the visual indications that I try to maintain as long as I am latched. Depending on the setup it can look different for other people. This is the standard configuration with TrackIR 5.

Finally the whole thing in motion:

As soon as your tanks are full the tanker will disconnect and you will receive the message "Transfer complete. Disconnect."
Reduce the throttle, turn off the exterior light if you have it on and close the RCVR. The READY light on the Canopy Rail Indicaty will stay on for up to three minutes after you close the RCVR

Congratualations, for the first time you have succesfully finished an aerial refueling!

I said it before and I will say it again. Getting good at this takes a lot of practice. Don't expect to be good in no time and don't give up when at first you fail. If it ends in flames don't worry about it, take a break and try again in the next mission. In the end it is just a game.

When I was training AAR I tried to refuel once or twice a week. It often took several reconnects to get the tanks filled, now I barely lose contact. I started about 6 months ago with simple formation flying and now I'm finally there. This guide is supposed to help you avoid the most common mistakes and give you a few tips to make things a little bit easier. It is not a substitute for practice!

Cheers, Derby ;)