Setting up credentials is in the main enterprise installation guide. You should have a Technical Lead within your organisation that has completed the Tech Lead training and should be able to answer this for you (our certified Partners will have tech leads that have completed the installation bp exam)
You create your encryption scheme as part of setting up the Blue Prism product - you can then forget about it. The 192 scheme is just there for backward compatibility but should not be used for new implementations, just select 256 stored on the application server and you will be good to go.