After you get your top pieced together, and ironed you are ready to sandwich the quilt. There are pros and cons to using a bed sheet for the backing. They are readily available, and big enough, but the thread-count and weave is not the same as what you have used for the top. If you are hand-quilting, the needle does not glide as smoothly.
At most fabric stores, there is a small selection of backing fabrics that are 108" wide (I think that's right). What I generally do is to buy the regular cotton fabric, and join it with 1 or perhaps 2 (equally spaced) seams. Be sure to iron them flat. Once the quilt is quilted, the seams are "invisible". The backing fabric, and batting should be several inches larger than the quilt top.
A word on batting: I prefer low-loft, but am in a warmer climate than you. I have found the high-loft more difficult to manage by hand. There are some wonderful products made with recycled materials, bamboo etc. Should your batting not be big enough, you can sew it together, but be careful to lay the pieces side by side and carefully whip-stitch. You don't want a big lump in your quilt.
Now for the sandwich: Some do this in the floor, but I am not agile enough, so I have used the dining room table, and sometimes the bed. Lay out your quilt-backing, right side down. Lay your batting on top, and then place your quilt top, right side up, on top of the batting. Make sure it is centered, and that you have extra backing and batting on all sides.
Start in the center, pinning the 3 layers together with large quilter's safety pins. It will take 100's of pins. Some baste it all together, but I do better with pins. Continue to smooth, and work your way towards the edges. Don't go all the way to the end in one direction, then start another. Make it more of a circular process. Keep smoothing constantly as you go. The key here is not to get any puckers on the back. My quilting looks more polished and smooth when I use a large quilting hoop, to keep the area tight. We'll talk more about hand-quilting when the time comes.
Fellow quilters, did I miss anything?
Patti, I hope you can picture how this all comes together. For me, sandwiching the quilt is one of the more difficult parts, I think because of the contortions involved. LOL You sew, girl!
BTW, a good source for supplies is www.keepsakequilting.com