Montana's election process is similar to Missouri's. There will be a primary but it will have ZERO effect on the election of delegates.
Each County GOP will hold County Conventions (although they are not technically called County Conventions) to elect delegates to the State Convention in June where national delegates will be elected.
I only found information for one County Convention (Glacier County), which will be held on May 15. That means these conventions are either happening now or will be happening very soon.
To participate and vote in the County Conventions, you MUST be a Precinct Committeeman or Committeewoman. If you are not a PC, there is a slight chance you can still become one but you MUST show up to your County Convention. If there is a vacancy in your precinct for a PC, you can ask to be appointed.
Contact your County GOP to find out when they will be holding their County Convention. If they say there is no County Convention, ask when they will be electing delegates to the State Convention.