While Service Pack installations should be quite straight forward, there is some general advice to keep in mind:
The smaller the step, the easier the upgrade. I usually recommend to integrate new SPs as soon as possible and don't leave any SP out. It's easier and less effort to go from SP7 to SP8 than going from SP1 to SP8.
Check the release notes of the service pack to get an overview of included changes, fixes, etc.
If you need to know more details, you can also look at the contents and coverage of the SP package. Sometimes it's easier to just install the SP on a dedicated testing environment and compare it to a non-upgraded one.
The most interesting part are project-specific overlays and other customizations from the project. I recommend to keep a list of all product customizations and have them checked by specific tests on each upgrade. This may be a bit of work why the overall recommendation is to keep the list short, meaning to customize as little as possible.
Regarding your second question, AEMs Pattern Detector can help because it helps you to identify deprecated or discouraged patterns in your code base. It is commonly used during upgrades of mayor versions (e. g. migrating from AEM 6.4 to 6.5) but may help you for SP upgrades and the quality of your code base in general. If you want to go one step further and prepare for a potential future upgrade to AEM as a Cloud Service, you may also want to have a look at AEMs Best Practices Analyzer, that is built on top of Pattern Detector. That being said, neither Pattern Detector nor BPA are mandatory to use in the context of a SP installation.
2. Pattern detector main purpose is to find the detection rate when we are upgrading between the versions. Since the service pack will be installed on the same version , pattern detector will not be needed