Help Manually Bleeding Brakes

Can someone tell me how to manually bleed the brakes on my 1983/733i. I changed pad on all four wheels and thought that I had blld the brakes correctly. But now (sometimes) when I start the car the brake light comes on and the peddle goes to the floor. I can rev the engine for a sec or two and the brake light goes out and I have brakes. They are other times when I'm driving and have brake, then suddenly the brake light will con on and no brakes for a few seconds. I'm assumming the brakes needs to be bleed. Is there anything else that would cause this problem. I have plenty of fluid in the resovire.

Thanks for your advice in advance