WRITE MUSIC

Assuming that capturing a keyboard performance isn't an option, getting a "natural" performance out of samples, regardless of their quality, involves a lot of micromanaging of the notes' timing (both start and end) and Velocity values, in addition to using the various contoller values that the software may support. Those controller values are usually of the MIDI variety, with the most common and important ones being Continuous Controllers (known as MIDI CCs) for things like Expression and Modulation.

Unfortunately, not all software supports these kinds of things. Of the freeware I can remember sampling, Musescore only really supported velocity; LMMS had velocity and timing, but seemed to lack standard controller support; and Sekaiju supported all of that, but was purely a MIDI editor (so, no fancy software synthesizers or soundfonts without workarounds). Granted, I'm pretty out of the loop on music software these days...

Without knowing what it is you're using, this is the most I can say on this, so hopefully that helps (and wasn't too confusing).