Overtime & Double Overtime - IF function

I have a spreadsheet that calulates the Overtime & Double Overtime on a daily input basis. However, the formulas are incorrect and I cannot seem to get them to work correctly. Please help!!!

Standard Time = 0-8 hours
Overtime = 8-12 hours
Double Overtime = 12-24hrs

Right now, the spreadsheet calculates the time as follows:

Standard Time = 0-8 hours
Overtime = 8-16 hours
Double Overtime = 16-24hrs