The most common approach is to record in stereo using two condenser microphones. The AKG 414 is a preferred microphone because of its low end. You can also use a Neumann KM184. If you are on a budget then use an AKG C-1000 or AKG C-3000. These sound pretty close to the AKG 414. I consider these a poor man’s version of the 414. You can get good results using those microphones too. So in conclusion the AKG C-1000 or C-3000 are good microphones. AKG 414 is a better microphone.
Try to use two of the same kind of microphones. If you are recording a baby grand or a grand piano. Raise the Piano lid and place the microphones inside close to the strings. Place one by the bass strings and one by the treble strings. It is important to keep the microphones on the same plane or level to avoid phase cancellation problems. It is also important to keep the microphones separated by a distance of a couple of feet to avoid phase cancellation issues.
You should know that when you use two different sources to record one location you may encounter phase anomalies. This means that the recorded signal will arrive at two different times to the audio interface. The lower bass will cut out and you may hear a comb filter effect. This can be addressed through microphone placement that is done before you record your tracks. If you discover the problem in a signal that is already recorded then you can also hit the phase switch on your console or audio interface. You can also hit the phase switch from within your DAW. This is only a solution if you discover this after the audio was recorded and the session has ended.
This can be done in two ways. You can use the stereo line outs and go into a compressor. Then run the signal through your audio interface.
You can also use the midi part of your keyboard and record a midi track. The keyboard has to be capable of being a sound module so you can access the sounds in the keyboard for later retrieval. This is because when you record using midi you are just recording data. You don’t record actual audio. Some keyboards will not allow you to access their sounds in a DAW environment.
There are some software synthesizers out there as well. You may be able to use your keyboard as a midi controller and play sounds from your software synth. When I purchased Sonar X3 it shipped with several software synths. Using your keyboard to access those synths really expands the capabilities of your keyboard.