You should have 90-degree angles at both shoulder and elbow and the pads should be just under/slightly forward of your elbows.
But if you've just fitted clip-on bars to a standard road bike, this might not be possible.
A specific TT or tri frame has a lower front end, a greater angled seat-tube and a shorter top tube than a standard road frame - alternatively, some people use a smaller size road frame as a TT bike, or you can fit a forward-offset seatpost and a shorter, down-sloping stem to get the same effect.
These changes allow you to keep the same hip angle between your legs and your torso as on a road bike, but the position is rotated around the bottom bracket
- think of the bottom bracket as a fixed pivot point
- drop the height of the bars, move the saddle forward and shorten the top tube, the rider will pivot forward and down, keeping the same angle at the hip
But if instead you just fit long aerobars and have to lean forward further to use them, you’re closing up this hip angle and also straining your back – not ideal for power and making an uncomfortable position it’s difficult to maintain
But this assumes you're making a dedicated TT bike.
If this is your only bike and you're going to use it as a roadbike as well, it would be best to adjust the aerobars to be as short as possible (or use some of the shorty bars like Spinacci's) and move the saddle forward on its rails.
You’ll have to accept that the pads will be under your forearms rather than your elbows and you will not have your shoulders and elbows at 90-degrees but your arms will be stretched-out in front of you, so it won’t be comfortable.
You won’t get the front end as low as it should be for best aerodynamics, but if you did you’d have a position that, unless you’re extremely flexible, would be very uncomfortable indeed for anything longer than a 10
As for elbows width apart, it depends on your shoulder size and on aerodynamics.
If you have your elbows touching, you present a block to the air, whereas if they are apart the air can flow between them down your chest.
But too far apart will increase drag, so it’s somewhere in the middle, perhaps with your elbows at knee width apart so they break the airflow to your knees.
However, the further they are apart the more stable you are – so I’d recommend initially you have them really wide, ride the bike like that to start with, with it as stable as it can be until you get your confidence up, and then when you’re happy with it you bring them closer together.
