This shows how drives actually mount to the plastic trays.
The 3.5" HDDs will mount quite easily - there are plastic tabs where the screws would go so you can actually mount these without any tools at all. If you want the drive to be even more secure or you wind up breaking the tabs, there are also 4 screws on the bottom that line up to the bottom mounting holes on a standard 3.5" SATA drive.
You can also mount 2.5" drives (SSD or HDD) in the same plastic tray. However, you'll have to use screws to attach the drive to the tray. There are 4 holes that will correspond to the bottom mounting holes on a standard 2.5" SATA drive.