If you don't have Waves Platform, please download the waves platform first: https://wavesplatform.com/
Create New Wallet in Waves Platform
Buy yourself some Waves from any exchange, I would recommend Changelly , or ShapeShift
Click on the "Waves Decentralized Exchange" as shown below
INPUT "COEVAL" ON THE LEFT TEXTBOX, as shown below
INPUT "WAVES" ON THE RIGHT BOX. Or if you have Bitcoin, you can just input BTC.
Scroll down, and you have option to BUY or SELL Coeval, as shown here
Input any amount you like, you get charged 0.003 waves for each successful transaction.
Happy Trading!