The first way that Shelley builds suspense is by delaying resolution to the conflict. The monster wants Victor to make him a companion. Victor at first refuses, but finally agrees. They part company and Victor goes back to Geneva with the family. We as readers must wait - will Victor go through with this plan? Will the monster honor his agreement to leave with the companion?
The doubt increases for Victor, and for us. He can't find the courage to go through with the plan, and so we begin to feel anxious for him. What will the monster do? His trip to Scotland makes our anxiety more acute. Shelley uses description to heighten the mood, and the wild landscape of Scotland is dark and foreboding:
It was a place fitted for such a work, being hardly more than a rock, whose high sides were continually beaten upon by the waves. The soil was barren, scarcely affording pasture for a few miserable cows, and oatmeal for its inhabitants, which consisted of five persons, whose gaunt and scraggy limbs gave tokens of their miserable fare. Vegetables and bread, when they indulged in such luxuries, and even fresh water, was to be procured from the mainland, which was about five miles distant. On the whole island there were but three miserable huts, and one of these was vacant when I arrived. This I hired.
He is alone, surrounded by nothingness. We might as well be in a horror movie. However, he has at last started his project, so we have the hope that he will get himself out of danger. That sense of hope amidst the scary backdrop makes the suspense worse - will he survive or won't he?
This is increased by the appearance of the creature in the window, and the destruction of the creature. But again, instead of having a confrontation between Victor and the creature, Shelley delays this moment by having the monster run off in anger. What will happen?
Description and plot delay - this is how Shelley heightens the suspense.