Double declutch downshift (assuming simultaneous braking):
1. Brake
2. Clutch in, select neutral, clutch out
3. Still on the brakes, blip throttle to rev match
4. Clutch in, select gear, blip throttle to rev match
5. Clutch out
6. Repeat Steps 2-5 as necessary
Standard heel 'n toe
1. Brake
2. Still on brakes, clutch in, select gear, and blip throttle simulatenously
3. Clutch out
4. Repeat 2 and 3 as necessary
Double declutching is a lot more work, it takes more time (not faster despite what Vin Diesel says!), but it will probably be a lot gentler on your synchos in the long run.