What are you having trouble with specifically?
Put the back lever down into the slot in the airbrush body, put the trigger in and then screw the tube shank assembly in from the back. Then insert the needle, tighten the needle shank and put the handle back on.
Why are you removing the back lever and all anyhow?
There is no reason to remove these parts normally.
“Wisdom is the right use of knowledge. To know is not
to be wise. Many men know a great deal, and are all the greater fools
for it. There is no fool so great a fool as a knowing fool. But to know
how to use knowledge is to have wisdom. " Charles Spurgeon