look in the database and locate the record for the username and password of the particular user you are doing this for, make note of the value for the UserID primary key column.
Go To Modify -> Security Assist -> Access Rules Manager
Create a new rule, set the condition to allow if.
Click the lightning bolt next to value and select the UserID session variable from the dynamic data bindings.
set the criteria to equals.
set the compare to value to the UserID value you noted from the database in the beginning.
on the pages that will be restricted to this user only, go to insert -> SecurityAssist -> Access Rules Manager to apply the access rule.