In very simple words:
Just download the Waves Wallet or Waves Client, Search for Dex Icon / Button, then search for GreenHashes in the list of Assets, Buy GH using any of 8 the supported Cryptocurrencies...

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

GreenHashes are available for purchase on the DEX Exchange in the Waves Wallet / Platform.
Asset ID for GreenHashes is: 7HTcrvUgnPjrXzApXWBWN3Q5dottvtun9iFM8KMdM2rD

Procedure to Trade GreenHashes on Mobile:

1) Install Waves Wallet 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 Dex Decentralised Exchange by clicking the Dex 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 Client from https://wavesplatform.com
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 Dex 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.