It depends on what kind of information you want to keep and how much time you want to devote to it. It can all be done easily on the computer. If you're interested in expenses, mileage, etc. perhaps a spreadsheet is all you need.
If you just want to 'talk' about your trip, jot down things as you're doing your daily travel and when you have time, elaborate more on them within a word processing document. You don't have to write about each and every day - just times when they were meaningful for you and you want to remember them.
I also kept a easy four column table document of campgrounds we stayed or of others' recommendations or places I read about. The columns were with headings of State, City (nearby), Campground Name and in the last column I gave my impression of it, would we return?, cost, ease of fitting our 40' in it, etc. - usually just 3 or 4 lines of text. I was able to sort this BIG list by the first three columns so it was easy to find things. We full-timed for 16 years and I made an entry for each place we stopped, including boondocking. Friends have asked me many times for information from it and sometimes I'd even print out a 'state' for them if that's where they were traveling. Many recommended that I sell my list. :) To me, it was very valuable over the years as we returned to many of the same areas.