After waiting until the vulcanizing fluid is DRY (about 5 minutes), apply the patch, center to center. The fluid is not a glue (which bonds when wet), but a surface preparation that is complete when dry. Press well. I leave the plastic backing on so the prepped part of the tube won't bind to the tire casing, and so the patch doesn't stretch with the rest of the tube, possibly stressing the repair. And because I'm lazy.
This is the 4th patch on this particular tube. Very serious time and money savings over paying a shop to replace the tube ever dang time! Replace the tube when there's no more room for patches. The Dunlop valve favored in the Netherlands remains popular there because the valve seal can be replaced independently of the tube, again without removing the wheel, so a tube's life can extend to decades.