Basically, there's two ways that I know of to reliably scan negatives...
Plan A: Buy/rent/borrow a flatbed scanner that has a built in light in the cover to effectively light the negative up...just like making a contact print. The key here is the 'right' light...wb, intensity, duration that's 'known' to the scanning hardware/firmware/software. Coming up with a makeshift 'flat' holder for the negative(s) as well as proper exposure issues may work so-so to 'good enough' for one or two negatives, but after 10 or more, you really need a proper negative-capable scanner.
As an aside, slide scanning has the same limitations. The same flatbed scanner should have attachments for slides as well. Some dedicated slide scanners have auto-feed stackers,too!
Plan B: send them out to be scanned, or pay a friend that has the capabilities.