In the search box
Alternatively select All mail from your labels list, you may have to click the drop down triangle in "More..." To make it visible.
This search will not include trashed emails, if you want to include them first untrash them or use the search
in:all AND is:trashed
tip: you can use any search directives (what you would type on the search box) directly on the "has the words:" field, and it will override any selections made on the filter creation form! just test the search and click "create filter with this search", your arbitrarily complex search directive will generate a filter.