1. Place the tip of your tongue behind your upper front teeth.
2. Curl the sides of your tongue up towards the roof of your mouth.
3. Blow air forcefully out of your mouth while keeping your tongue curled.
4. The combination of the curled tongue and the airflow will create a vibrating sound, which is the trilled "r" sound.
5. Practice pronouncing the sound by itself until you become comfortable with it.
When pronouncing "rra" in Setswana, it's important to maintain the strong trill throughout the entire sound. It can be helpful to listen to recordings of native Setswana speakers pronouncing the letter "r" to get a better understanding of how to produce the sound correctly. With practice, you should be able to pronounce "rra" accurately and fluently.