To answer your question directly Sheira (as in He-Man?):
I prefer screws, but some people use velcro, and others use doublesided sticky tape.
Jokes aside. The question is obviously
1. what do you want to do with them (=removable or not)
2. how skilled are you?
The easiest (fixed) solution is to get some 3.5 to 2.5" adapter.
which fits two: one on the surface with the screws into the underside of the SSD/hdd; and the second with the screws into the side. There are cheaper adpters out there but they are typically just bent sheet metal (=look tacky).
Speaking of the screws into the underside of the SSD. If you have the skill to drill four holes correctly spaced, some modders have mounted the SSD to the extra width of the motherboard tray.
Finally, removable: There are several brands essentially making the same mechanism. Here the Rosewill
example as 3.5" bay for two 2.5" drives. I have the Silverstone Tek version (TS03) wich combines one 2.5", a slim DVD slot and two USB ports. This thing is also available as PCI slot mount. Quite nice because it allows an extremely short SATA data cable from the mainboard.
Removing the PCI hardware, it can even be mounted underneath the motherboard tray..........