As others have suggested: Start by breaking down the task into the smallest part possible.

The absolute best way of estimating development task we've seen was suggested by PHK (of FreeBSD fame), or at least I remember it as one of his suggestions. Basically you take your absolute best guess on a task, multiple that by two, and move up one unit of time.

Example: My guess for task X is 2 hours.

2 hours times 2 is 4 hours

Move up a unit, that's 4 days.

My guess for two hours can realistically be completed in 4 days.

This prevents you from taking on to large tasks, guessing that you could bang out a project in 5 weeks would realistically be a 10 month project. A two year project will be done in 40 years (4 decades).

It not a realistic way of estimating time of cause, but it illustrates the dangers of tackling to large tasks.