In very simple words:
Just download the Waves Exchange Application on Mobile or Desktop or Laptop or simply go to https://waves.exchange, create an account, then search for GreenHashes in the list of Assets, Buy GH using any of the supported Cryptocurrencies...!

For people interested in the minute details & a step by step guide...

GreenHashes are available for purchase on the Waves Exchange. https://waves.exchange
Asset ID for GreenHashes is: 7HTcrvUgnPjrXzApXWBWN3Q5dottvtun9iFM8KMdM2rD

Extremely important note for old Waves Dex Users.
Waves Platform has discontinued Dex (Waves Decentralised Exchange Product). You have to use the Waves Exchange App from Mobile or PC or Mac. https://waves.exchange

Procedure to Trade GreenHashes on Mobile:

1) Install Waves Exchange App from Apple's App Store / Google's Play Store.
2) Set up User Name & Password. Then take proper backup of the Wallet by writing down the Seed / Backup Phrase.
3) Fund the Wallet with some Waves (for Fees) & Cryptocurrency Of your choice that you wanna use to Buy GreenHashes. (Bitcoin / Litecoin / Ethereum /
Bitcoin Cash / Dash / Monero /  ZCash & Waves)
4) Go to Waves Exchange by clicking the Exchange button on right side bottom.
5) Click the + Button on Right side Top. Then click on Options on Right side top.
6) Click “Show Unverified Assets”
7) Refresh the list couple of times. Type GreenHashes & select it in Search Bar.
8) Refresh the list & Search again if GreenHashes is not visible. Select all the GreenHashes pairs.
9) Go to HomeScreen & you can now Place Buy / Sell Order as per your choice.

Procedure to Trade GreenHashes on Desktop / Laptop:

1) Install Waves Exchange from Waves Exchange Website.
2) Set up User Name & Password.
3) Take proper backup of the Wallet by writing down the Seed / Backup Phrase securely.
4) Fund the Wallet with some Waves & Crypto that you wanna use to Buy GreenHashes. (Bitcoin / Litecoin / Ethereum /
Bitcoin Cash / Dash / Monero /  ZCash & Waves)
5) Go to Exchange by clicking the Exchange Button.
6) Select Cryptocurrency Of your Choice for buying GreenHashes.
7) Select GreenHashes in Search Bar.
8) Click Buy / Sell.
9) Place Buy / Sell Order as per your choice.