Its easier to start you pressure flaking using the bottom of wine bottles or other thick glass material (bottom of vases, etc). After attending a couple of days with Karl Lee, I definitely found that it was so much easier to get the technique right using glass initially and then moving onto flint.
You'll find that glass flakes easier and uses much less pressure compared to flint (which can be a real bugger).
The other bonus is that you now have an excuse to empty all the wine bottles and to buy plenty more!!!
well thats only partialy true most beer bottles are slightly curved on there bottoms which makes them somewhat tricky to knap. 90 % of the time you can get it to work without any problems and make a nice arrow head that is razor sharp but it will always be some what curved from the bottle.