Maybe too much detail, but hope this helps...
Submitting card images is one of my favorite parts of the site. I guess I just like feeling like I'm contributing something too.
I recently bought an inexpensive Cannon LIDE 300 scanner to scan cards and I've been pretty happy with it. I use the built-in scan utility and set the scanning size for:
- Standard cards: 2.45" x 3.45"
- Relic cards (in the default slab you get from Topps): 2.85" x 4.15"
and those size settings seem to work really well. I have it set to open a 'preview' when done scanning that opens the scan in the default Windows 10 Photos app and it has a button on the toolbar to easily rotate cards and auto-saves them as rotated, which is quick & easy. I rename the files, probably not necessary, and then submit them to upload.
I've only been doing this for a few weeks, but the biggest issue I've had so far have are dealing with the reflective cards like Finest, Chrome, etc. and keeping the card and scanner bed as dust-free as possible, which can be a bit challenging if you're picky about it. :)