It was not that difficult, although I made a mistake...
2 - 4 Stunden
Speed up my Mac mini with a Crucial M4 SSD, and extending hard disk drive space (1TB Samsung)
Unfortunately I first did not place the second hard drive (that one where the original drive was) in the right way, so I had to tear down again although I was nearly ready to close the mini...
After doing this again (and positioning the hard drive correctly), all went as it has to be...
I needed one evening (about 2 to 3 hours).
Be sure you have installed the upper hard drive correctly (screws must fit into holes!), otherwise you have to to tear down again;)
Also the power cables should be in the right place.