Launch your DAW program, such as Ableton Live, Steinberg Cubase or Reaper. Click 'Start," "All Programs," and then select the application from the list that comes up on the right-side of your screen. Open the audio project that needs mastering by clicking "File," "Open project" and selecting it from the list of saved project files.
Add a four-band equalizer effect to the "Master" output buss on your DAW's mixer. Set a low-pass band at 30Hz, reduce a second band's peak at 500Hz and then place a high-pass band at 15kHz to roll off any unnecessary high frequencies. These EQ settings give an entire mix a natural flow, comparable to how other professional mixes are prepared for home listening and even club play.
Add a multiband compressor after the EQ on the DAW's "Master" channel buss. Raise the "Attack" and "Release" levels to high setting, and then reduce the ratios and gain reduction levels. Adjust these settings if it changes the overall sound of the original mixdown too much. Turn off the compressor effect at times to audition the original version against the one with the mastering effect.
Add reverb and minimal delay effects to the "Master" output channel buss on the DAW's mixer in order to give the final mix added spacial complexity. Keep the mix levels on both types of effects at minimum, otherwise they could overpower the mix.
Render your mix and compare it to other professional recordings. Verify that all levels sound the same between tracks, and that your final mastered mix does not sound over-produced and transparent. Go back into your DAW and make changes to the mastering effects' settings to fine-tune your final version of your rap song as well.