How to to use a scale to compose melody bass line or riff?

I chose my key then make scale on the area i want on the piano or guitar and then i get confused on the process of using it. Im guessing its just playing the tonic root note to the octave over until you hear or the note you want and start building on to rhe melody your creating. Or use modes to change the root note so you don't have to start or go back to the tonic with out changing the key or interval patterns and use chords or arpeggios inside them and eventualy it will the notes will come natrualy so you can play freestyle and shred. Is this the way to use it. Please not to cmplicated