I generally lie sideways on my bed, with my legs hanging over the edge. I put the brief/diaper down on the bed, then transfer from my wheelchair. As I'm transferring I try to center myself on the diaper as much as possible before sitting down, then pull it between my legs and tape/pin.
In a public restroom, it's a bit more challenging. I'll lay the brief on the wheelchair seat and transfer from the toilet. Once in the wheelchair seat I try to come as close to lying down as I can, then I pull the brief up between my legs and tape it up. Centering can be a good deal harder, since I have so much less room to move around. Invariably I have to adjust the brief an inch or two to the left or right before I can tape it on.