Above all, aspiring instrumental rock guitarists should listen to lots of instrumental rock bands and musicians where the focus is on the guitar. This gives you the opportunity to hear how different players use the guitar to make music people want to listen to. Listen to what scales players use, how they texture their guitar sounds and what kind of sounds the players use to evoke different feelings in the listener. In a band with a vocalist, the words and vocal tones set the mood in a straightforward manner. Instrumental rock guitar is much more nuanced and subtle. Listen to a broad array of instrumental rock, from surf to shred.
Knowing as many guitar scales as possible provides you with a number of templates to pull from. As an instrumental rock guitarist, you won't be merely reproducing scales in song form. However, you also won't reinvent the wheel. Surf guitar uses a lot of Middle Eastern-influenced scales, while shred relies heavily on classical music scales. The more scales you know, the more creative you can be when crafting songs and solos. Learn scales from a broad array of musical genres, from classical to jazz to blues.
Effects take the clean tone of a guitar and make it into something else entirely. Effects pedals range from simple (chorus, reverb and compression) to otherworldly (such as pedals that emulate torn speakers and the myriad of metal distortion pedals available). Stacking pedals together allows you to create a sound that is truly distinctive. Shop around for different pedals, spend time playing with them and buy anything you like the sound of. An instrumental rock guitar player should have a large array of pedals at his disposal.
Experimentation allows you as an instrumental rock guitarist to find your individual sound. Try stacking different pedals together and seeing how they sound. Combine scales that don't seem to go together, like jazz scales and classical scales. Recording helps you examine your experimentation by keeping a document of what you did. This allows you to listen to what worked and what didn't. Remember that at the end of the day, the only person who has to be satisfied with your playing is you. Experimentation allows you to come up with a sound that is distinctively yours.