Andrea, you do not "make a critical path". The criticality is determined by the inputs to your schedule of
- activity durations,
- logic between activites,
- calendar work periods and
- any constraints placed upon activity dates.
CPM calculates the total flioat for all activties in the schedule network, and criticality is determined by the value of the resulting total float for activities.
If the critical path that is created is not what you expect it to be, you need to check how you've built the schedule based on the items I've listed above. IF you need to drive criticality through a certain path, it may be that you need to apply a date constraint to the additional path - which is indicating that you need to have completed that sequence of tasks by a fixed date.