So do I reassure him with my words from my own bed or by the crib?
I'd try and do it from your own bed, with a view to being out of the room before he falls to sleep. If his crying is escalating, you can go to the crib and try to soothe him. Both of mine were really hard to calm down at this age, but at least being by the crib they knew they weren't alone and they did eventually go to sleep.
Also, is there a link that explains how to do gradual withdrawal step by step? That's the same thing as gradual weaning correct?
I thought there was one but had a quick look around and can't find it. Basically you will try to calm him by patting him, or stroking his head, or whatever, and when he calms down you gradually stop what you're doing and take a step away from the crib. If he starts to escalate, you go back to him, if he stays the same or calms down you take another step away.
He has a cold and a bad cough but no fever, do I wait until he's better to start?
I'd wait, so you feel confident and not feel like you should stop sleep training.