A Tower Local Controller reported they forgot to issue appropriate control instructions to an arriving aircraft and it flew below the minimum safe altitude and initiated a turn away from terrain.

Narrative

I was sequencing VFR aircraft into Runway 17. Aircraft X was inbound from the northwest to land. I advised them to fly over the approach ends of Runways 16R and 17 at 6;500 feet with no further instructions. I sequenced the aircraft in front of Aircraft X north over interstate 15; and then local center started to coordinate a release for one of my departures. When I came back to Aircraft X I thought I had issued them instructions to overfly interstate 15 as well; but I had not. Aircraft X advised they were going to turn for terrain east of the airport. I then realized I had not given Aircraft X any course to fly and rectified that prior to entering an area with a MSA above the altitude Aircraft X was at. Suggestion: I got sidetracked with coordinating with local center and when that was done I had thought I already gave further instructions when I had not. In hindsight I should have had local center coordinate with me later so the sequence in what I wanted to do wasn't interrupted. In the future I will also give complete specific instructions to aircraft on what to do to enter the pattern right away so they know what the entire route to the runway will be and I will know I already gave those instructions.