I have not done this specific project, but I think the idea is to set 8 'row' pins high on your PIC, and set 8 'column' pins low. Scan all pins rapidly. You are looking for current leaving a 'row' pin, and entering a 'column' pin. (In itself this is risky, and you need to ensure what are safe connections regarding amount of current, what resistor values are needed, etc.)
Each unique combination goes with a particular keypress. You have a chart already made.
Create a 2-dimensional array for a string variable A$. Set the variables:
A$(1,2)="Q"
A$(1,3)="W"
Etc.
Suppose you detect a keypress at row 1 and column 2. Your program tells you which character via the command:
character$ = A$(row,column)
or
character$ = A$(1,2)