If I declared income I didn't actually receive, do I need to declare the full amount when I did or just what I haven't already declared before?

[SweetieJean]

You declared that income for 2017?

[atomic_comics]

I declared 75% of what I received over the last couple of years. Only 25% is new income I had not previously declared before.

[atomic_comics]

In other words, I didn't realize they hadn't paid me and declared what they would have, and then they paid me a lump sum in 2018 including what I was due for that year.

[Bsch4477]

Assuming you operate on a cash basis you report income when it is received.  If you reported income on past returns that you didn’t receive you should amend those returns and report for 2018 what you received in 2018.