During my first year as a thrifty college student, I purchased a device known as the "trim comb". Basically a wide razor blade between comb teeth.
To shorten one's hair, you hold it just so, and lightly comb downward. Hairs will be cut. Repeat as needed. Inevitably it will catch on the back of your head in line with your ear lobes, so you compensate by attempting to make a smooth transition line a half-inch higher. After several such attempts, the hair on the back of your head looks like patchwork.
Eventually I decided my budget could include a trip to the barber from time to time. Such is the price of vanity.