Two methods:
1. Cuttings of firm ripe wood (preferrably from a main shoot) about 10 - 12 inches long taken in late September/early October. Trim off immediately below a leaf node and remove all the leaves (but not the side buds) from the lower half. If there is a growth bud on the top nip it off. Soil not too heavy, and lay a line of sharp sand along the line where you intend to insert the cuttings. Make the holes with a stick (in the centre of the line of sand so that some dribbles into each hole) to a depth equal to the denuded lower stem, stick in the cuttings and firm around. Leave for at least 12 months, or longer if growth is poor.
2. Bud onto briar or rugosa stock like the nurserymen do. This will give you bigger plants more quickly, but is only really necessary in the case of very weak growers. I used to do this in my younger days, and can give you full instructions if you wish to try this, but it takes a bit of practice to get it right, and you need to get hold of some seedling briars of the correct stock.