Purchase a 808 drum kit sound pack. The dirty south style is based primarily on the sounds that came from the Roland TR 808 keyboard years ago. You can find these sounds from various websites that sell equipment and sounds to hip-hop producers. Look for an 808 kit that is compatible with your beat making software. A good kit can run about $30.
Open up your beat making software and load your 808 drum kit into the step sequencer. You will need a kick drum, an 808 snare, hi hat, and an 808 clap.
Set the tempo for your dirty south beat. The beats per minute (BPM) vary greatly in the dirty south style of rap, but the most commonly used BPM range is from 75 to 89 BPM.
Arrange a drum pattern. Have your kick drum hit on the first beat of the measure, the snare on the fourth beat, and then the kick hits again on the first beat and the third beat. Have your hi hat hit on every beat of the measure. Create variations of the drum beat using the clap and by also changing up the hi hat pattern from hitting on every measure to hitting every other measure.
Load your synthesizer sounds into the program and create a melody for the beat. Have the first layer of melody be a deep, almost bass-like synth sound and then lay a melody over top of the bass. You may also want to use some horn sounds and some strings to arrange a melody.
Arrange your beat into sections. You will need to have 16 bar verses, 8 bar choruses, and at least one bridge section for your song.