Risks that exist from this exercise are: burst blood vessels, sometimes causing bleeding from the urethra (which can lead to infection and other complications if not taken care of), and temporary or even permanent erectile dysfunction.
There are many products (tools, instructions, etc.) that one can buy; however, much of what they offer (instructions) is available on forums and free websites.
Jelqing is typically preceded by a warm-up with a hot compress on the genitals, and concluded the same way. The reasoning behind the 'warm-up' mimics the normal operation of the biological processes of the penis during the four stages of arousal. The third stage is heat. At this stage the smooth muscles of the 'glans' can easily expand to their present (not future) greatest potential size.