You can book in-person appointments up to 42 days in advance. All the information can be found here:
https://www.gov.uk/ukvi-premium-service-centresWhen you go to book, there is a point in the scheduling that will direct you to the IHS website. You will need to pay the IHS fee at that time and then enter the number produced in order to proceed with the booking.
You a centre choose from a variety of locations, then the day, and then the appointment time. If I recall, they were spaced about every 10 minutes apart. Then, once you have chosen the time, you pay the visa fee which is the charge for the FLR(M) extension + in-person fee.
Once all that has been done, you will be sent a checklist with all the information about your appointment which must be printed and presented when you check in for your appoint (at Croydon we arrived 20 minutes early and they made us return when it was 15 minutes before, don't know if the same is true for Glasgow). You will also want proof of IHS payment and, of course, your FLR(M) form.
So, you will have to pay the IHS charge before you can even look at appointments, but this payment must be done before sending the postal application anyways. My suggestion would be to be ready to schedule 42 days before your desired in-person appointment date and see what they have available. I scheduled my 10 May appointment on 3 April and there were plenty of options still available at Croydon. Hopefully the same holds true for Glasgow. Good luck!