autopilot help

well, sad as it is , im still confused by the autopilot. i once remember being in the air after take off, and fooling around with it, and i think i definitely pressed Nav hold, and the plane literally flew itself, following my flight plan i previously got from the route. i'm having trouble doing this again. Soemtimes i use the Heading hold switch, and that works for a short period then it stops following? what am i doing wrong? sorry if this was asked before :[

thanks in advance

i also would like to know what the difference is between GPS and Nav Mode switching. ty

You need to set up a flight plan using the FS Flight Planner. This will create a pink line in your GPS from your originating airport to your destination airport.

Then, when you takeoff, switch the GPS/NAV switch to GPS and press NAV on the MCP (Autopilot Control Panel). The aircraft will follow the line to your destination airport.

The GPS/NAV switch is a sim feature that allows you to choose whether you want the AP to respond via the onboard GPS or via external navigation i.e. ADF/NDB, VOR, ILS.