The built-in way
Sort it by date added, then work from the bottom
- Open Downloads in Finder (⌥⌘L) and switch to list view with ⌘2.
- Choose View › Show View Options and tick "Date Added". Click the Date Added column to sort by it, oldest at the bottom.
- Start with the oldest files. Drag anything you want to keep into a folder (Documents, Pictures, a project folder).
- Select what you don't need and press ⌘⌫ to move it to the Trash.