Do-it-yourself climbing wall

What do I need to figure out before I build my climbing wall?
Which boards should I choose for my climbing wall?
How do I make climbing boards myself?
How many climbing holds should I put up per square metre?
Can I use MDF or OSB boards for climbing walls?
Which climbing holds should I choose for children?
What is the difference between different types of climbing holds?
What about fall surfaces and safety of climbing walls for children?