How do I do the spinal decompression therapy at home with no devices?

Perhaps you can. If you have good hand and arm strength you could hang from a pull-up bar and let the weight of your trunk and legs put traction on your low back, which may feel good. If you try this, be careful getting up and down, and be careful not to fall. The bar would be equipment. There may be a beam or bar that is part of your living unit, but you need to be sure it will support your total body weight.
Any way of hanging . Upside down like off the side of your bed otherwise if you could purchase something it would be a lumbar traction device or an inversion table.