You cannot snap and app on your console whilst exporting else it will cancelTo create a clip press and hold

and either press

to start a recording or

to turn on action replay mode which records in the background. You can record clips from Story and GTA Online.
Clips must be at least 3 seconds long, once you're happy hold

again and press

to save the recording. The shorter the clip, the quicker the export.
You're now ready to export,
whilst in story mode press

and scroll right using

to the 'Rockstar Editor' menu and press

to enter. Note the Rockstar Editor is only available from story mode. From the Rockstar Editor mode 'Create new project' which will load the editor and then add your clip from the 'Add clip' option.
Press

to exit the timeline and then you're ready to export the clip, choose a name and confirm you wish to export.
Once exported the achievement is yours.
Export another 9 clips (can use the same one from above) to unlock
Heres a video