Without some explanation of the specific scenarios you're looking for it's difficult to give you any "step by step".
User Group Policy application is influenced by:
- The Distinguished Name (DN) of the user object in Active Directory and GPlink objects in Organization Unit objects in the parent path of the user object DN
- GPlink objects at the AD Site object containing the IP address of the computer where the user logon occurs
- The "Block Inheritance" attribute of OUs in the parent path of the DN of the user object or at the AD site object
- The "Enforced" / "No Override" attribute of GPlink objects that are linked to OUs in the parent path of the user object or at the AD site object
- The security group membership of user accounts and the permission that confers upon them with respect to applying the GPOs or applying specific policy or preference settings within the GPO that might have ACLs attached to them (Software Installation policy and Group Policy Preference settings have ACLs within the GPO, for example)
- Enabling Loopback Group Policy Processing on computers (in either Merge or Replace mode)
Those are all the mechanisms for controlling user Group Policy application. Between all those functions you can create reasonably complex deployment scenarios.
Your OU hierarchy should always be designed first based on planned delegation of control. You only get one OU hierarchy and there are no good mechanisms to decouple delegation of control from the OU hierarchy. Design for delegation of control first, Group Policy application second.
I focus on using security group membership to filter user Group Policy application. Never forget that some settings within GPOs have ACLs separate from the GPO itself. The "Block Inheritance" and "Enforced" / "No Override" functions should be used sparingly and are typically indicative of a broken design. Loopback Group Policy Processing can be a very powerful and useful tool, but it's a bit complex to understand.
You're focusing on users but I'll take a moment to mention computers, too. Computer Group Policy application has all the same filtering functionality as user Group Policy, but also includes WMI Filtering. WMI Filtering can let you target GPOs to specific hardware or operating system-related attributes of computers. I often see security group filtering neglected when filtering computer Group Policy.
There are some things you can accomplish with both WMI Filtering and security group filtering for computer Group Policy. WMI Filtering has the added feature of being dynamically calculated on each Group Policy application (versus security group memberships, which must be changed manually to affect filtering). Security group filtering can still be beneficial if you have computers located in disparate OUs without a WMI filterable attribute that need to have common Group Policy Objects applied. I use security group filtering with computers, frequently, to control Software Installation policy (wherein I have a GPO with multiple software packages assigned, each with a unique ACL that includes a group to control the software installation).
The most important thing you can do, setting aside everything I said above, is to make sure you fully understand how the Group Policy client calculates the Resultant Set of Policy and, with that knowledge, brainstorm and test possible designs before putting them into production. I firmly believe that testing should start with pen and paper (whiteboard, etc) rather than software. You need to have enough organizational knowledge to be able to design realistic scenarios and test them. Get other groups in your IT organization (and outside the IT organization if need be) involved. Your Helpdesk team, for example, will have delegation of control needs that will drive the physical OU hierarchy. Your desktop support team may have software installation needs that drive computer group policy filtering. There are any number of possible scenarios that should be talked through, worked out on paper, and finally tested in a lab scenario before being put into production.