Im having so much trouble with it. I just can’t get the yoyo to land on the right string. It’s landing in between the strings or the wrong string. Any tips please?
When the string comes around your non-throwhand index finger for the first time, try to catch the string as far back on your index finger as you can. If you have trouble with that, you can try pointing your finger up so the string slides back into that position. When the string comes around your non-throwhand index finger again, try to let the string come around it further up your finger so that the strings are spaced out. The closer your finger is to the yoyo, the easier it is to catch. Just put your finger under the yoyo and it will land on the string.
Hope that helps. Just keep practicing and you will get it. 
Just keep training. I did. Before you notice it, you got your dubble or nothing;)
Make sure hat the yoyo is landing rather close to your finger when you’re learning, much like when you started learning trapeze and would land the yoyo close to your finger. Also, like stated before, have the first wrap far back on your finger
Another thing you can try is to start by learning Houdini mount (where the first wrap goes around your thumb instead of your index finger). Learn that really well and then go back to Double or nothing. It teaches you the motion and the feel of the trick. And for me it made double or nothing easier to do.
Space out the front string from the other two.