You can't. There is no offline play mode.
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go on the freaking website and where it says play PLAY IT! it will validate it, it will take a few minutes because your on the website then it'll show up somewhere and u can play it.....
Enter no password, and the options will appear of 'play offline' or 'try again'. Select play offline.
It's currently impossible to play RuneScape offline.
You can not play combat training offline.