to design 10ft * 5ft banner of actual size what is the exact process for down scaling illustrator and raster images in photoshop please guide me i am new
It greatly depends on the end use of the banner, the printer it is being printed on, the substrate, and the company you are working with.
The newer printers (in the last 10ish years) can print 1200 x 1200 DPI, some of them even higher at good quality and color. If it's a sports field sponsor banner, doesn't really matter. If it's a trade show banner that could be looked at from a foot or 2 away, the higher the quality the better.
10' x 5' is not too large even for older versions of illustrator. That is a standard flat bed printer size. Since boards like that aren't usually seen from that far away (not like a billboard), I wouldn't scale it down at all.
But if you must, I would only go down 1:2 at the smallest, and still size your photos to at least 300 DPI. If you do scale it, make sure you communicate with your sign company on what your final size is, and what scale you are sending them to make sure that there are no miscommunications.