Q.Pruning camelias japonica & Susanqua
how to prune a 7 ft tall camellia into shape of a tree?

Here are some articles that will help:
http://extension.oregonstate.edu/gardening/node/398
https://www.americancamellias.com/care-culture-resources/general-culture-requirements/pruning-camellias
Pruning a large camellia into a single-trunk tree can be done, but it may be difficult depending on how it has grown so far. It can be easier to convert an overgrown camellia into a tree form with multiple trunks- you would still remove all the lower branches that come off the chosen trunks.
They can also be cut back severely, in case you want to effectively start again:
https://www.rhs.org.uk/advice/profile?PID=194