The groups (pairs/quads/octets) that we form can also overlap each other. It means that a single value can be contained in more than one group.
To reduce a Boolean expression using K-map, first make the K-map for the expression. Now group the 1s in the priority order— Octet quad pair single value. That is, make octets wherever possible than quads than pairs. After that, remove the redundant groups. Then write the reduced terms for the remaining groups and add them all. The result is your reduced expression.
Here's a list of all the tutorials we currently have in this area - Introductory Digital Electronic Circuits and Boolean logic
Tutorials in Basic Digital Electronics: At a glance