look for a instruction sheet that should come with the box/bag/package
usually, you just put it on where you want it, put a cloth or something on top, and just iron around a bit.
theres usually glue on the backside of the patch, so the ironing would melt the glue, to make it stick.... after a while of ironing, if it doesn't stick at all, that means theres no glue. so you should either get a patch gluing kit, or stitch it on with a needle.