An activity log is just a record of how you use your time over a period of a week. It will help you identify when you used your time well and also identify times when you could improve your study efficiency.
Try and include the following in your activity log:
record every change in activity
include all activities including sleeping, eating, family time, studying, social activities
write down how you feel because this will identify times when you may study most efficiently
By analysing the activity log you will be able to identify both good and poor use of time and you well be able to make use of many extra hours that have previously been wasted.