Your users are under Admin then Users. Users who are trainers will have a calendar icon beside their names. Employees who you have indicated are not trainers will not have this button. You can always change the status of your users by clicking the pen icon to edit them: 

You can access the trainer availability setting tool by clicking the calendar icon beside your users that are trainers: 

On the next screen you will be shown a calendar for the week. You can switch which week you are viewing the same as in calendar week view by clicking the two buttons that the orange arrow is pointing to below. From here, you click on the time and day you wish to set an availability timeslot for and you will be asked if you wish to set a recurring availability (an availability for the same time every week) or a single-day availability (only for that day): 

After you choose if you wish to set a recurring availability or a single day availability, you will be asked to set the timeslot, time between bookings (a buffer time between two appointments in that timeslot), and a location (a location must be selected.)
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You will also have the option to add another time slot to create multiple availabilities within the one day with one simple step: 

This will then show on the calendar like this:
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Go to the "Calendar" menu, "View Calendar", then click the "Trainers" tab: 

You will then be taken to the trainer availability tab where you will be able to see the trainers, what times they have allocated that they are available, and which location they are available at: 

You can add an appointment the regular way by clicking the orange "+New" or by clicking the block of time in which you can then set the appointment for the time within the block:

Then when you click "Submit" the appointment will show on your regular calendar and also on your Trainers calendar: