Modern Quilt Group
Free Tutorial - Layering Your Quilt

It's really not a quilt until it's quilted

So, in order to make it into a quilt, and not just a Top or a WIP (work in progress) you need to go back to the quilt shop and invest in some batting/wadding and a backing fabric.
Now, you cannot do better than a natural fibre (cotton or bamboo) batting, preferably a thin one. Polyester is much harder to quilt and may well not be nearly as nice to sleep under. And cheaper is not better, in this case - you spent hundreds of hours on that top - give it some respect, please...

You can make backings from narrower fabrics, but there's a lot to be said for the extra-wide ones when you are either in-a-hurry or inexperienced..

Your backing needs to be BIGGER than the front, by at least 4 inches each way for all but the teeniest quilts..
The top, however carefully you quilt, may "walk" a little, and it will walk right off the backing and batting if these are too small.

A note on pieced backings - I adore these, and they use up odd lengths of fabrics nicely, but you cannot match front and back seams. (Nor should you, as it makes for needle-breaking bulk.) Just accept that the front is where you see the stitching.
For wallhangings I generally use a good soft calico (Ikea) as it is plain, cheap, handles nicely and washes if required..
Bed quilts need something nicer.

If your quilting skills (or your tension-adjusting) need work, use a patterned back!
(Note - I don't like sheets for backings, as they tend to be too closely-woven and can be stiff in use. Polycotton is utterly silly - it lasts much less well than the top and is horrible to quilt...

Now, the first thing is to get your backing nice and flat, press, press, and lay it out Right Side Down. Pin or tape it down so that it does not move, but is not under tension (or it will spring back). You will note that I am working on a carpet-covered table above - this is probably my most favourite tool, but if you cannot make one of these and don't like crawling on the floor, then consider investing in trestle tables, or renting a village hall for a day with your buddies and layering lots of quilts...

Next, shake your batting/wadding out nicely and gently, and smooth it gently on top of the backing. Then the (well-pressed, for the last time) top. Smooth gently into place, working from the middle outwards, and making sure it looks square and not crinkled anywhere.

I pin small quilts with straight pins, but anything over 36 inches in any direction with good quality safety-pins (cheap ones don't seem to have points any more). You need LOTS of these.
Pin at least every 4-5 inches; every time you put your hand on the quilt you should cover a pin. Start in the middle, and pin to either edge, then across the other way, then fill in working outwards all the the time until you hit the edges. Pin from the top, if you run your hands underneath it will make crinkles...

When all is done, turn it over to check your pins have gone through and proceed to Quilt!

I'm not a fan of holding pinned-but-not-quilted quilts for any length of time - unless you really have to do lots at once, it's better to layer, then quilt at once

All this seem too hard? Most long-arm quilters will baste a quilt for you to finish - this is absolutely Not Cheating...

See also
Double Bindings
Flange Binding
Packing Your Quilt to Send it Away
Blocking Your Quilt

Coming soon
Odd Shaped Bindings
Quilting Your Quilt