There are two ways to approach this....on a RIB the first way is not entirely practical, but I will explain anyway. Most ski (GRP-type) instruction boats have a bar which is mounted off to one side, approximately amidships, protruding about 5ft outwards. All one does is hold on to the bar with the instructor next to you teaching you the correct posture, leg position etc.This is pretty easy if you have a semblance of balance.
The other way is a bit harder and that is the traditional way of 'just doing it!' A lot depends on the skill of the person at the helm. Once in the water with the your feet snuggly in the bindings, adopt a position of going to the bogs. Arms should be outstretched and skis aproximately 6 inches apart. The ski-rope should be kept taut at all times by the helmsman. Once you are more or less ready (you will find that you never are quite ready), signal to the helper on board, a thumbs up as opposed to a shout/scream, which can be mis-construed. Keep your arms nice and straight and NEVER pull yourself up. Once the tension becomes unbearable, proceed to 'get up'. Do not stand bolt upright as you will be pulled over. Concentrate on forcing your skis together as they will tend to want to pull apart.
RELAX and enjoy it. Always remember that the engine/boat is potent enough to pull you out, therefore it is not necessary to pull yourself up. Once underway raise and lower your arms to compensate for slackness in the ski-rope, NEVER pull the handle as you will be pulled out of the bindings as soon as the slack catches up with you.
remember to let go the handle if you do fall over. Hope this helps.