how do write-in candidates for congress get their name out to the voters?

Answer 1

By what is called a "Grass Roots" campaign

Grassroots organize and lobby through procedure including:

door-to-door, also known as canvassing
phone banking
house parties
meetings
putting up posters
talking with pedestrians on the street (often involving informational clipboards)
gathering signatures for petitions
setting up information tables
raising money from many small donors for political advertising or campaigns
organizing large demonstrations
asking individuals to submit opinions to media outlets and government officials
get out the vote activities which includes the practices of reminding people to vote and/or transporting them to polling places.

Answer 2

First check with the Board of Elections in your state. Ohio requires write-in candidates to register before the election. Votes for non-registered write-in candidates don't count.

Once you are sure your votes will count, you can spread the word however you want.