Plastic canvas come in ready-cut shapes, including crosses, or you can buy a piece and cut it to as fancy a cross shape as you desire. Plastic-canvas embroidery kits may come with yarn, and you can do stitches that are popular for cross-stitch, needlepoint and even crewel embroidery. For something a bit different, you can weave narrow satin ribbon through the canvas and itself, allowing the plastic frame to take a more prominent role in the craft.
Cut ribbon into workable lengths a few inches longer than one of the dimensions of the cross. Thread one piece on the needle.
Draw ribbon up in the centre of the horizontal part of the cross shape and pull it about halfway through. Push the needle back through the next space to the right and up again in the next. Continue out across the extended arm of the cross.
Re-thread the needle with the other half of that ribbon and weave it out in the opposite direction, following the same pattern.
Thread a second colour of ribbon on the needle, cut for the other dimension of the cross. Bring this up in the same space where you started the first ribbon and weave it out in perpendicular directions.
Weave the two colours in the two directions, working out from the centre, with each colour forming parallel sides of small squares. You'll see the grid of the canvas showing through between the squares.
Cut the ends of the ribbons off evenly along the edges, or whipstitch over the ends of the cross arms to finish them.
To achieve a tweed pattern in the central square, offset the stitches in each direction like bricks. Instead of squares, you'll wind up with two-color zigzags running diagonally. Leave the ribbon a little loose to cover more of the plastic canvas.
Be careful not to twist the ribbon as you stitch.