The inputs I used literally only work with this spawn point so I don't even know if it's worth explaining, but basically you want to get to x=2027.75 with x velocity -0.1, which you can get with 2 one frame left taps (50% chance of being the correct distance) plus a -0.8 pixel shuffle in the middle. From here, full flow to the right lets you hit 10.4 speed 0.05 pixels from the right wall, which is enough for a double.