Hello,

I just found out that I neglected to report capital loss carryover from six years ago up until now. Can I just begin reporting that loss now? (How would I do that?) OR do I need to amend those previous tax returns first??

Thanks for your help.

yes you must amend your previous 6 year returns and apply the capital loss carryover to tax years and then take the remaining, up to the maximum allowed, on your 2016 tax return.
To amend your tax return use Form 1040X