FIRST, WHEN TO WASH YOUR TENT:
It’s easy to store your gear away without a second look when you return from a trip. But the care you give it will return in dividends toward its longevity. Over time, dirt, smoke, sand and other natural elements can weaken a tent’s fabric and grate on its components, like zippers. Washing your breathes life back into it. An advertising tent is a big gear investment, so the longer it lasts, the better for you. Andy says consider washing your tent when:

It’s visually dirty: A little dust doesn’t warrant a wash after every trip, but if your tent looks particularly caked, or if you’ve allowed dirt to build up for a while, it’s time.
Water does not readily bead–up or the fabric wets–out easily: Grime degrades DWR (Durable Water Repellent) and waterproof coatings; removing that dirt layer and revitalizing them is important.
After camping at the ocean: Sand acts like micro abrasives on fabrics Andy says. In addition, salty air corrodes zipper sliders and aluminum tent poles, and sand can prevent pole ferrules from inserting fully. “A good soak and rinse is ok, but ground–in sand needs soap to break the surface tension of the dirt,” Andy says.
After prolonged exposure to campfire smoke: Smoke coats your tent in a layer of micro particles; do your tent well and remove them.
After prolonged UV exposure: UV rays act like an oven, baking dirt into the fabric. There’s no escaping UV exposure, but keeping your tent clean helps prolong its life under the sun.

Detailed steps are introduced
Fill the tub with enough cool water to completely submerge the tent body and rainfly.
Add a small amount of soap. The tent’s size and the type of dirt will dictate how much.
Agitate the water by hand. Turn the canopy/mesh inside out to remove dirt from pockets, gear lofts and corners.
Knead the tent, pushing down and swishing it around for about 5 minutes to ensure water reaches the nooks and crannies.
Let the tent soak for 20 minutes, then repeat the agitation cycle. 
Lift the tent and fly from the tub and observe the water. If the water’s brown, drain it, squeeze the water out of tent—do not twist it out—refill the tub and repeat the process. 
When washing is done, fill the tub with clean water and rinse the tent and fly in the same manner. 
Hang the clean tent over the shower rod, drape it over a clothes drying rack, or lay it on a clean surface. Avoid sharp corners and edges. Do not hang the tent by its corners because this puts stress on the fabric.
Ensure the fabric is “bone dry” before storing it away. “I personally leave my tent hanging and flip it inside out and over for at least five days,” Andy says.