In order to attach an SLA to one of your customers, first, open their profile page (either search for it or simply grab it from your customers list). Click into 'Quick links':
Click the 'Add/edit SLA' option from the 'Associate SLA' section. This will open a side panel from which you can choose the SLA you'd like to apply. If it is not available from the dropdown search, simply go to your SLAs and check that it has been saved correctly or add it there if need be.
Once you've selected the correct SLA, hit 'Save' in the bottom right in order to attach it to your customer. You can check that it has been successfully applied by going back to your customer's profile. In the 'Customer details' section, you should be able to see which (if any) SLA's are active on that customer's profile: