Linking two Look-up fields

I want a custom made look-up field on the Leads form to link directly to a custom Look-up field on the Opportunities form i.e. If a user adds a value in {Leads: Lookup} the same value is to appear within the {Opportunity; Lookup}, essentially they are the same field used across two different entities.

Is this possible? If so, what are the steps required to build this functionality?

(apologies in advance if this question has been answered before, I have searched the forums but could not find the answer).

Thanks