Original source on MSNBC:

https://www.msnbc.com/the-beat-with-ari/watch/see-trump-vet-roger-stone-pushing-elector-plot-on-tape-beat-exclusive-190982725545

Edit: Skip to 4m 35s for the intro to the video. Video starts at 5m 08s.

Video was taken November 5, 2020 before the election was even called (November 7, 2020). Roger Stone can be seen with an assistant dictating a letter explaining how alternate slates of electors could be used.

Transcribed it as best I could (pauses removed):

Roger Stone (slowly dictating a letter his assistant is seen typing up):

Although state officials in all 50 states must ultimately certify the results of the voting in their state, the final decision as to who the state legislatures authorize be sent to the electoral college is a decision made solely by the legislature.

Any legislative body may decide on the basis of overwhelming evidence of fraud to send electors to the electoral college who accurately reflect the president’s legitimately victory in their state which was illegally denied him through fraud.

We must be prepared to lobby our Republican legislatures by personal contact and by demonstrating the overwhelming will of the people in each state that this may need to happen.