At the end of the week, I look back at the hours/tasks I have allocated to me and I fill them in. Generally, there's no link between how long something took me and how many hours I write in it; I just write all the hours in. If there were 8 hours set aside for it, I write 8 hours in the box. Sometimes I spread it over several days.
If I don't have enough tasks/hours allocated, I look back at anything I did during the week that isn't on the allocated list, and I have it added to the allocated list and then I write the hours in. Sometimes, I'll do this during the week rather than all on Friday.
The purpose is to fill in the boxes and make the numbers match. Nobody ever examines the boxes and numbers, and they bear no relationship to what actually happens. If I don't fill in the boxes and make the numbers match, someone comes to insist that I fill in the boxes and make the numbers match.
In the ones that I get added, I pick the number. Other ones just appear with numbers already in them. Sometimes I like to spread the numbers out. Sometimes I just do them in big blocks.