Setting up Layout form of any entity, is very basic step when developing a form/Process for the client.
Salesforce provides vanilla layout and new fields are added automatically on the layout, but grouping, Layout and Read-only Behavior need to be edited as per the Business requirement.
so Lets start this customization step wise ,
1. Go to Page Layout Option , Setup -- Customize -- Entity (Opportunity in my case) -- Page Layout
2. Once page Layout is clicked the default layouts are visible for different applications, select the one for your Application and Click Edit,
There are option to remove a fields, Edit its property or Replace it to other place by click and Dragging it
4. Once done, Click on Page Layout Assignment and Edit Assignment,
Here assign the page to role as per requirement
Suppose You need to Assign the New Layout to Sales Profile then choose Sales in Profile and Select the Page Layout you Just Changed
very basic, yet Most Important :-)