That really depends on your budget.

What you need is a plasmid (a peace of DNA) with all of the machinery needed for cobalamin production on it. You can do in-silico modeling to get the plasmid sequence but then you will need to make it. In order to make a plasmid you can either do all the wetwork yourself, which requires a lot of equipment and reagents or you can pay a company to synthesize do it for you.

Once you have a plasmid the rest if pretty straightforward: get the plasmid into bacteria (can be done with ice and calcium salts), get the bacteria into a container with sterile media (a fancy chicken broth with salts) and let them rip :)

Finally, you will need to isolate your cobalamin form the used media, but you'll need to look into that, I haven't done it before (filtration and fractioning most likely) ;)

If you need specific guidance on the process and materials to do it at home then look into biohacking and DIY Science communities, maybe there is even a physical one somewhere nearby if you are lucky! ;)

Good luck and take care!