So not in a schedule then?
You already have the form almost correct. Using a checkbox name that's an array will allow you to retrieve the checked items out of it. So you have it named todo1. The value is incorrect though, make that whatever the primary key is for the record.
When you submit it, you can simply implode that and put it into an UPDATE. What you do exactly with that will depend on what the datatype of the primary key is.