Recording how you know something to be true in genealogy is very important. Many genealogists regret not doing so too late!
Sources in Pedigree Forge are entered and edited via the source editing panel, which can be accessed from the second button on the “Action Bar”, which runs down the left-hand side of Pedigree Forge’s main window:
At the top of the sources panel is a table showing all sources so far entered in the project (the list above is empty). Clicking on one of the sources will make it the active source and allow you to enter or edit details. The buttons along the top of the table allow you to add, delete, or change the order of sources.