How do you log ground time?
Just as it says in 61.51(b):
1) General--
(i) Date.
(ii) Total flight time or lesson time.
(iii) Location where the aircraft departed and arrived, or for lessons in a flight simulator or flight training device, the location where the lesson occurred.
(iv) Type and identification of aircraft, flight simulator, flight training device, or aviation training device, as appropriate. (v) The name of a safety pilot, if required by Sec. 91.109 of this chapter.
(2) Type of pilot experience or training--
(i) Solo.
(ii) Pilot in command.
(iii) Second in command.
(iv) Flight and ground training received from an authorized instructor. (v) Training received in a flight simulator, flight training device, or aviation training device from an authorized instructor.
(3) Conditions of flight--
(i) Day or night.
(ii) Actual instrument.
(iii) Simulated instrument conditions in flight, a flight simulator, flight training device, or aviation training device. (iv) Use of night vision goggles in an aircraft in flight, in a flight simulator, or in a flight training device.
So, when I give ground training, I log the date, the amount of time, and the subjects covered, plus the standard signature/CFI#/exp date.
Does the endorsement in the back cover it?
If you mean the flight review endorsement per AC 61-65, no, not unless you include the 61.51-mandated data in that endorsement, too, which I've never seen but I suppose is possible.
Do you use a line in the logbook? Training folder?
Most student/standard pilot logbooks have a section in the back for Ground Training, and if that's there, I use it. If not, I put it on a line in the logbook, entering the appropriate data. Putting it in a training folder doesn't help the trainee unless s/he, rather than the training provider, keeps the folder.
And yes, the FAA is getting sticky on this point, having instructed examiners to check that there is ground training logged in all the required areas per the appropriate section (e.g., 61.105 for PP). If those squares aren't all filled, they're supposed to cancel the test, send the applicant back for more logged training, and if it's a pattern with an instructor, notify the FSDO for further review of that instructor's practices.