Take a look at a youtube channel - Nick Carver. He is a professional architectural photographer that shoots large and medium frame film camera as a hobby. This means that he digitizes the negatives. He has quite a few videos on his techniques - and film holders with flat bed scanners - how he exactly goes about it, approaches, software, problems, and solutions. Also, his videos are extremely well done and entertaining.
Rather than posting links - which would just make this post long... here are some titles of the videos where he does some scanning - there are more, these are just a few that I can recall.
- Tips for Cleaning Dust on Film Scans
- Fine Art Photography: Ocotillo Prints and Ocotillo Prints Follow-Up - These two he used both flat bed and drum scanning
- Comparing Film Scans: Drum vs Flatbed vs DSLR
- Film Scanning Tips for Epson Perfection Scanners
- Film Photography Tips: Scanning Instant Film (without Newton Rings)
- Making a Fine Art Photography Print (1 of 2) - he starts out with scanning...
PS - I also recall that he also uses another technique with some glass plates and a couple of drops of liquid to hold the negative flat against the glass, then he needs to do some manipulations to get the flatbed scanner to focus on the plane of the negative - I think I remembering this correctly.