Conditioning shins is basically done by consistent training
The thing about thais kicking trees is kind of a misconsception thanks to the movie Kickboxer
Basically back in the olden days the thais would kick banan trees which were very soft anyway
You should never strike your shin with anything harder than the bone unless you're in a fight. Some people wach shins with sticks, glass bottles etc, well that's not great. In the short term you'll cause calcification over the shin but in the long term it will weaken the shins due to the occurrence of hairline fractures.
The best way to condition shins are the tried and tested methods which are:
Hitting the heavy bag. Hit the bag as hard as you can 100 kicks each leg every day
Hitting Thai Pads (The harder the better)
Bags come in different degrees of hardness at most gyms, go with what you can hit the hardest and feel pain then move on
Running is also important. The thais also have a way of running by which they run on tiptoe/ball of the foot. Basically it works the calf and shin muscles and strengthens them
Have you seen the size of some calf muscles of Thai boxers?
Well I hope that helps you on the conditioning part
As for stretching... Fyre wrote a great article on stretching so maybe i'll give her the Mic
