To qualify for almost all visas in Rwanda, they are going to ask you for a police clearance from your last residential living location from the past six months. You can still come to Rwanda and then apply online from your previous location to get it however, some people rather bring it with them when they travel over.
The process is simple, however it varies from state to state and country to country. The key thing is to first research your last residential location to find their website and contact information. Reach out to them and follow the guidelines they specify. There is almost always a cost, but it varies depending on your state and/or country.
To give an example, here is a video of how to get a police clearance from the United States. This video shows you the step by step process.