It's actually very simple to get a Rwandan e-visa. Here is how;
Fill out the application form online and pay the visa fees using PayPal or your credit card. Application fees are $50 for single entry visas and $70 for multiple entry visas.
Once you've submitted your e-visa application you'll receive a confirmatory email, answer the questions asked and submit a scanned copy of your passport and a passport-sized photo.
You'll receive your visa upon arrival, the visa is valid for 30 days but citizens of COMESA states can get visas that are valid for up to 90 days.
Important info
Since you'll get issued with the visa upon arrival you can only enter Rwanda at these specific entry points.