1. Create a prefilled button on your Custom object or on a Standard object
The path to the button is different for Custom and Standard objects.
1a Custom object:
Setup > Build > Create > Objects > Your object > Buttons, Links, and Actions
1b Standard object:
Setup > Build > Customize > Your object > Buttons, Links, and Actions
2. After the above steps proceed with:
Fill in the snippet and paste it in the text box.
3. Repeat step 1 and 2 from the main manual for your prefilled button.