As Keith said it is all practice, you will eventually develop an ear for the right revs for the changes. Once you get the hang of it you wont forget, I learnt to drive a crash box when I was seven, by the time I got my licence we had updated to synchro boxes in the truck but 30 years later when I got the blitz it all came back in a couple of miles. Down changing while going up hill is easier to master than when you are going down hill. Enjoy a long drive and have fun learning.