How to estimate LOE for a project or feature

keep following things in mind:

1. Everyone is good at estimating the small tasks which are always not bigger than 3 days.
2. Big project can be divided into media tasks/features.
3. Features can be divided into small tasks
4. Tasks can be several types
   1. Design -> Input: Reviewed FDD, Output: TDD
   2. Coding -> Input: TDD, Output: the product feature Dev version
   3. Unit test -> Input: The product feature Dev version, Output: The product feature Unit test Version
   4. Buddy test -> Input: The product feature Unit test Version Output: The product feature Buddy test Version
5. Finally roll up all the LOEs for each tasks. If the LOE for some tasks are bigger than 5 days, normally that means it can be divided into more tiny tasks/features.