One of several ways.
1) Go to Documents\Flight Simulator X files and start FSx by double clicking on 707-300 Cold-n-Dark (KSEA) or 707-300 Cleared for takeoff (KSEA)
2) Start FSX and Load 707-300 Cold-n-Dark (KSEA) or 707-300 Cleared for takeoff (KSEA), then click Fly Now.
3) I start from the Free Flight with the 707-300 at whatever ever airport I choose and start a flight that way.
4) After flying with another aircraft, change over to the 707-300 and continue flying from there.

These are probably all the ways to start a flight. Sometimes things go wrong when I use method 4. Switching from one payware aircraft to another can cause a fatal error in FSX. Not often, but it does happen.

Mark