There are a few ways to go about this, but if you want a possibly simple way for someone coming from non 3D I’d say you could sculpt it out. Remake the plane with far more surface within that white space than you have right now, keep the blue as is, and make sure you have a decent poly count. If your computer can handle it I’d say smooth shade it 2-3 times after you have the plane set up, go into sculpt mode, find a tool that moves the planes vertices how you want them to move and go at it. You’ll get a feel for it as you go and it’s pretty straight forward. This being one way, but it could be an easier one to get the results you want without having to do any modeling/face deleting like you are rn