To start I made a quilt sandwich as above.
Because this quilt will be machine quilted, I decided to baste using curved safety pins which is much quicker than thread basting. Without lifting it I stuck a curved safety pin in the middle of the quilt sandwich through the top, bating, and backing.
I added more safety pins in a spiral, placing them a fist's width apart until filled up the space.
Here it is completely basted.