Force.com Utility Belt plugin

step 1: open the url "https://chrome.google.com/webstore/detail/forcecom-utility-belt/bchgkjmjnmekbampjoenadmoekocpbhp?hl=en" in google chrome.
The page will be as shown in image below.

step 2:Add this plugin to Google Chrome.
step 3:Once plugin is added, open your instance.In the top right side,you can find red color "s" symbol.Click this icon and explore the related links.

show checkboxes in related list

If we want to display checkboxes in related list, Enable "Display Checkboxes(for MultiRecord Selection) " checkbox while creating custom list button as shown in image below.

The output will be as shown in the image below.

How to create a custom button in related list

step 1: Go to the related list object where we need to create custom button.
For example if we want to create a custom button in "contact" related list of Account detail page,
go to contact object and create custom list button as shown in the image below.

step 2: Go to edit layout of parent object(here Account) and edit the contact related section as shown in the images below