In the blog or chat script, you can point their database to the one store all forum's tables and change their table that hold account information to the forum one (so that account info is from one source). I not actually combine blog and forum together, but the concept should be the same.

I don't think there is any tutorial out there as this is a very customise 'project'. You may want to spend $100 or so to get a freelancer to do the job for you. Experience programer may take 3-5 days to complete the job. Try google 'freelancer'. Some site let you to accept bids from programmer that can do your job and may ended up very low cost.

All the best.