It sounds like this is your first trip to Europe. You do need to consult the Rick Steves guidebooks on Britain & Ireland. They will also include a lot of general information on European travel. You can see what they include by going to Guidebooks: Britain & Ireland and clicking on the ones of interest.
The basics of European travel are covered in greater detail in his Europe Through the Back Door.
In the meantime, there are resources right here on this Rick Steves website that can answer many of the questions you are asking and provide info on lots of details you don't know to ask about.
The standard phrase here is that there's a lot you don’t know that you don’t know. I've been traveling for about 45 years and even lived in Germany for 3, but I still have forehead slapping moments and learn something new every time I plan a trip
About European travel in general, go to Travel Tips. There you can learn about planning your trip, transportation, packing, money, phones & technology, theft & scams, sleeping & eating, health & hygiene and sightseeing & activities.
About the places you want to visit, go to Explore Europe. There you can browse countries including Ireland which includes Northern Ireland, Scotland, Wales and England. Each one has information on cities and locations, what to see there and more. Above I linked to the section on Scotland as an example.
As for going by ferry, there are several options. I've gone from Scotland to Belfast that way. It's a fairly short crossing, but going from Belfast to Scotland you need to be aware that it's still a relatively long train ride to Edinburgh. As others have suggested it might be faster to fly there from the Republic of Ireland or Northern Ireland, if that's possible. Your departure airport would depend on flight availability and/or where you finish up your Ireland visit.
If you're still interested in taking the ferry, the main route is from Belfast to Cairnryan. This is a link to more information about that route.
It doesn't matter how long or short your trip is, there will still be many logistical details to work out. Sometimes they will affect where you actually can go during the time you are there.