Read the twain spec. Then read the spec again. Consider using a prebuilt library to talk to twain_32. Dosadi does an excellent job. I used their free classic library several years ago and liked it. I never tried their pro library, but I hear good things. If I still had my vb6 code you would be more than welcome to it but I don’t know where it is, sorry.
There are a couple of articles for twain in vb6 over on planet-source-code for twain from vb6 and like everything else google is your friend. If you run into specifc problems try posting them here.