In Idaho to appear on a party's Presidential Primary ballot, a candidate must pay a "filing fee" of $1,000 to the Secretary of State.

Who said money couldn't buy Democracy?

A candidate must file and pay by December 11, 2019

Idaho holds a closed primary - which means you must register to vote as a Republican or if you're already registered  file a "party affiliation declaration form" - The last day to accomplish this is unknown.

Register to Vote! or update your "affiliation"