How are air ducts for furnace/air conditioner cleaned?

Unnecessary if your HVAC system is maintained and serviced properly which includes washing or replacing the filter monthly. Only needed if there has been recent construction, pest infestation, mold. Other than that it is a waste of hard earned money.

https://www.todayshomeowner.com/hvac-duct-cleaning-scam-or-worth-it/

https://www.epa.gov/indoor-air-quality-iaq/should-you-have-air-ducts-your-home-cleaned

In between service visits, you can do this:

https://www.softscrub.com/your-house-your-home/how-clean-heating-and-air-conditioning-ducts-yourself/

1. You need to vacuum them out. Turn off the power connected to the heating and air conditioning system [source: Reliable Remodeler].
2. Unscrew the air duct covers or grilles from the walls. Use your brush to cleanthe grates thoroughly. ...
3. Vacuum the ducts as much as possible. ...
4. Clean the grilles in your ceiling.

are you asking how you can do it? or how a professional actually does it? What to Expect From an Air Duct Cleaning Service Provider

If you choose to have your ducts cleaned, the service provider should:

Open access ports or doors to allow the entire system to be cleaned and inspected

Inspect the system before cleaning to be sure that there are no asbestos-containing materials (e.g., insulation, register boots, etc.) in the heating and cooling system. Asbestos-containing materials require specialized procedures and should not be disturbed or removed except by specially trained and equipped contractors.

Use vacuum equipment that exhausts particles outside of the home or use only high-efficiency particle air (HEPA) vacuuming equipment if the vacuum exhausts inside the home

Protect carpet and household furnishings during cleaning.

Use well-controlled brushing of duct surfaces in conjunction with contact vacuum cleaning to dislodge dust and other particles.

Use only soft-bristled brushes for fiberglass duct board and sheet metal ducts internally lined with fiberglass. (Although flex duct can also be cleaned using soft-bristled brushes, it can be more economical to simply replace accessible flex duct.)

Take care to protect the duct work, including sealing and re-insulating any access holes the service provider may have made or used so they are airtight.

Follow NADCA's standards for air duct cleaning and NAIMA's recommended practice for ducts containing fiber glass lining or constructed of fiber glass duct board. https://www.epa.gov/indoor-air-quality-iaq/should-you-have-air-ducts-your-home-cleaned

I agree with Naomie. It's a total scam. Replace the filter monthly and your good. Use pleated filters not the cheap blue mesh ones. That's what they do. Duct cleaning gets the contractor in the door to "find" other things "wrong". Dee has the right idea. I just dust my grills when they get dirty. Was in the HVAC business for 13+ years. Heard bragging stories from contractors how they ripped people off. The stories I could tell.