Use external view to get into correct position then Shift + U at same time, launch bar should lower, press shift + I at same time, if your close to where you should be, you will attach to the catapult, you most likely see your plane jump slightly, the exhaust blast barriers will raise behind you.
on Acceleration F18, use half flaps, increase throttle until afterburners come on, then reduce enough so they turn off.
Press Shift + spacebar at same time, this will launch you forward, once your clear of the carrier, climb away and immediatley raise your gear, if the gear won't raise, keep speed to around 200 knts for example, press shift + U again then try G for gear again. gear will not raise with launch bar down, don't forget to raise flaps or you may get flaps failure.
On the carrier deck you cannot steer with launch bar down.
If you have FXS gold installed, look at the mission where you have to do a protection of NASA'a cape area, you'll see you have wingman, you may see where he/she positions on the cat.
If your still having problem, what aircraft are you using and where did you get it from?
I could do some screenshots for where you should be on the cat, look at the channel in the deck for the cat, you might see a 'lump' sticking out of the deck, your nosegear should be just in front of it before you lower the lauch bar because of the lack of nosewheel steering.
You can lower the lauch from the cockpit on the F18, it's on the lower left side, if you scan the panel whilst you press shift + U you'll see which lever it is.
Don't for your gear and Hook ( shift +Q ) for landing.
Try not to have your nose too high on touch down, my frequent mistake is this, you catch the wire then it slams your nose into the deck.
You might find it worthwhile doing touch and goes on the carrier if the approach doesn't look right at the last second, raise your hook though
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Good luck.
regards
Steve.