How can I install my sata 3 hdd in a sata 2 port?

Welcome to the community, Kostas!

Just like @elbert mentioned the SATA connections are backward compatible, you will face no issues or performance bottlenecking if you install a SATA III mechanical hard drive on a SATA II motherboard. Moreover, an HDD is limited in the throughput by the platter density and its RPM and they cannot exceed the bandwidth of a SATA II port anyhow.

Keep in mind that if you plan to upgrade your primary storage to an SSD in the future, you will face some bottlenecking due to the fact that a solid-state drive is capable of much faster transfer speed ( ~ 500 MB/s). This is where a SATA II ( 3 Gb/s = 300 MB/s) port won't be able to provide the necessary high bandwidth which SATA III-compatible (6 Gb/s = 600 MB/s) mobos possess.

If you need the step-by-steps on how to physically install your HDD, you can follow the detailed guide in this tutorial from our KB.

Hope this helps you. Let us know if you have any additional questions.
SuperSoph_WD :)