Using it the first time surely be a bit hard, but like any other things after using it for some time, you would find it a must have tool everytime you play Dota.
So here's a simple tutorial on how to use Dota Hotkeys:
1. Download it if you don't have it yet on your PC.
2. Open it, you can open warcraft or dota hotkeys first as you like.
3. Then just press ALT+ the letter on your keyboard corresponding to the item on your inventory.
Refer to the image below: