I did not claim my RRSP contributions for Jan.& Feb. 2018 on my 2017 return. How do I claim them on my 2018 return?

You can't. You have to file an amendment to your 2017 return.

You can do that online if you have signed up for CRA my account, otherwise there form can be found here:

https://www.canada.ca/en/revenue-agency/services/forms-publications/forms/t1-adj.html