When buying canvas you will probably be asked several questions:-

TIES. Do you want TIES? These are canvas ribbons sewn along the top of the cloth about a foot apart and used to tie the canvas to the top batten or barrel. Ties are unnecessary if you plan to nail the canvas permanently to a top wooden batten. Ties will be an additional cost. You can just see the ties on the top of the cloth in this photo (opens in separate window)

POCKET. Do you want a chain pocket? This is a large hem along the bottom of the canvas and is used to thread a heavy chain through to help weigh the cloth down to remove any wrinkles and keep it taught. A chain pocket is unnecessary if you plan to nail the bottom of the canvas to a wooden batten. Beware of using a square batten on which to roll up the cloth as it will leave horizontal creases resulting in a Venetian blind effect. A chain pocket will be an additional cost.

HEMS Do you want the sides hemmed? As well as neatening the edges and stopping them fraying, hems will also help to alleviate the edges curling inwards. (I've seen this referred to as “hour-glassing”) Hems will be an additional cost.

STRETCHING . Once you've taken possession of your canvas backcloth I recommend that you stretch and seal it. Ideally this should be done on a frame. I normally build mine from 2inch by 2inch sawn timbre. Thick wood, as the canvas will be under great tension once sealed. But you can make a frame out of anything to hand. I have even used two heavy bookflats to hold the canvas taught at either side. And of course you can use the same frame to hang your canvases on when on the stage. Usually, where convenient, I will screw the frame to a wall, but it can be free standing and left leaning against a wall or hung from a bar in the ceiling.
Here is PDF of a suggested paint frame.(opens in a separate window)

One thing to watch out for is to keep any supporting bars which run across the frame well back from the canvas. If you have a wooden strut flat against the back of the canvas, any brushing across it will leave a line. Rather like a brass rubbing.

There is no reason why you can't seal a canvas whilst it is lying on the floor. Put down covering over the floor first of course. It is still advisable to keep it taught by nailing it to the floor. You can walk on the canvas when you are sealing it but of course not on the piece you have sealed and is wet.

SEALING When making up a sealing solution for a cloth I use very cheap white vinyl emulsion as my base and add a little PVA glue plus a small amount of red emulsion and a little water. The red is there for two reasons. Firstly when stirring up the solution, once it has turned a nice light pink I know it has been thoroughly mixed and secondly, once applied, this gives the canvas an overall light pink colour which helps me see the chalk marks when marking out and transferring from the original sketch. (Although lately I have been using charcoal to mark out) Red also makes a nice under colour for blue sky . Mix up more than you need.

Try to do the sealing in one operation. If there is a break it is possible the canvas will start drying and the tensions will be different when you start again. I have lost a foot in length from one side of a cloth when the painters used a different solution half way though the session.

Hope the above is of use.