Take a picture.
I'd love to replace my mini's HDD with an SSD but all the steps keep me from attempting it.
I just looked at a guide, it doesn't seem that bad at all assuming you have the right screwdrivers on hand. At least you don't need a putty knife anymore :|
edit: About the drive not fitting, does Apple use the 7mm drives or 9.5mm? That might explain why things aren't fitting.