As far as a guide for juniors, Ralph Maltby wrote the attached about fitting juniors and it is still applicable today. Take a look at the link below:
As far as building components, there are a lot of tutorials about building clubs online. We have a few basic assembly guides on our you tube channel, but nothing specific to building junior clubs. The assembly process is the same. The important thing is to get the proper components and build them to the correct length and overall weight. Too long or too heavy will be more difficult for them to swing, meaning less success. You might also consider a junior pre-built set. The quality has improved and they are a great value for juniors. The link below is to Golf Galaxy’s options for juniors.
I’m all for picking components and building yourself, but the value of buying a junior set for specific age / height and getting the bag and accessories is worth a look, in my opinion.