Forget the guide books at the moment as they can't keep up with all the developments as a result of coronavirus.... Your best source of information is the web or Orklney Tourist Office. I have found them extrememely helpful and they have the answer to every question you might think of. Details of contacting them here.
There is information about getting to Orkney here.
There are two main ferry companies, North Link and Pentland. At the moment, services are restricted to essential travel only. February and March are low season and boats are removed from service for scheduled maintenance, resulting in fewer sailings.
If you are in Abderdeen on a Thursday, Saturday or Sunday, you can catch the North Link Ferry which leaves Aberdeen at 5pm and arrives in Kirkwall at 11pm. DON'T worry about the late arrival as hotels, B&B and s/c properties are quite used to guest arriving at this time! Make sure you have a taxi booked to collect you from the ferry terminal.
An alternative is to drive north to Scrabster (near Thurso) and catch the North Link ferry from there to Stromness. However this would either mean the 6.30am ferry or one departing at 8.45pm...
Timetables for both here.
Tariffs here (But these are for 2021, so may well increase for 2022).
Pentland ferries sail from Gill's Bay, near John o Groats to St Margaret's Hope on South Ronaldsay which is at the southern end of the Orkney islands. It is the shortest crossing. Boats depart Gill's Bay at 9.30am, 1.30pm and 6.45pm. At the moment the 1.30pm service is suspended as one of their boats is in dry dock for maintenmance.