There is some good documentation regarding SLA's in JIRA including a few knowlege base articles with examples such as Example: creating a basic SLA, Example: creating an SLA with multiple cycles and Example: creating an SLA that doesn't track continuous time that can help you create your first SLA:
Those are a few resources available. In regards to using the SLA's in Extension for Service Desk, I have not tested this as that is an add-on created by InTENSO IT Experts. The only thing I found doing a quick search was from a product page:
This feature allows to calculate SLA metrics on the basis of data from custom fields.
You can definitely test in a staging instance or email firstname.lastname@example.org for further information about the operation of their add-on.
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