From conversations with my good friend @herrb:

• smaller things are easier to estimate so try to come up with any subtasks you can remember
• sort the list so it makes sense, e.g. polish the interface AFTER having functionality working
• revisit your estimates every now and then
• don’t remove your old estimates but add a new estimate column in your spreadsheet and see how the estimates change. This should help you getting a feel for where you mis-estimate.
• very important for estimates, updates, and monitoring: keep track of how much time you have spent on the tasks.