1. Create a custom button on your Custom object, or to customize a button on a Standard object
For a Custom Object follow:
Setup > Build > Create > Objects > Your object > Buttons, Links, and Actions
For a Standard Object:
Setup > Build > Customize > Your object > Buttons, Links, and Actions
2. After the above steps proceed with:
3. Repeat step 1 and 2 from this manual for your custom button.