Practice

Short answer: Two ways. Fly the official missions at home whenever you like, and fly practice runs on SFL servers during the practice window once your account is verified and you are 18 or older. You don't need to enter first.

• At home: download the mission files from your pilot home and fly them in DCS. See How do I install the mission files?.
• On SFL servers: practice is open to verified pilots aged 18 or older who have accepted the current registration documents, whether or not they have entered. It runs first-come, first-served, against a published per-pilot practice allowance. Practice continues in parallel with the qualification window, and qualification runs take priority when the two compete for server time. To book a qualification run, you enter and accept the SFL Pilot Terms and Conditions.
• Practice on SFL servers is recorded. SFL clips practice sessions and posts them to its channels, identifying you by callsign. The publication rule for pilots under 18 or not yet verified as 18 or older still applies (see Is there a minimum age?).

Source: Appendix B §B.2.4 and §B.9.1 (Appendix B tab) · SFL Media Release, §5

Short answer: No. Practice runs on SFL servers are scored so you can see how you did, but no practice run counts toward qualification. Flights at home are not scored at all.

• Practice scores on SFL servers are visible to you. They are not official scores.
• SFL does not score ACMI files you record at home. Official and ranked scoring happens only on SFL servers.

Source: Appendix B §B.2.4 (Appendix B tab) · the README.txt inside the mission files