To trim a hedge and get it straight, you'll want to set up stakes and strings, to guide you. Essentially, you "outline" the shape you want the hedge to take; then you simply stay within the bounds you've set up, as you trim.
But before we begin, I want to inject a sobering thought: it's not as easy as pounding the stakes into the ground, running the strings between the stakes, and then racing through the hedge with your hedge shears (as one might see in a cartoon)!
The problem is, the branches of the shrubs will keep getting in the way of your string, as you try to get a nice, taut line between two of your stakes. That's why I list "pruning shears" as a tool you need (see prior page), in addition to "hedge shears."
Even though the hedge shears get all the glory, you'll want to keep the pruners handy, too, throughout this operation. Using the pruners, you can remove the "worst offenders" -- i.e., branches sticking way out -- as you begin to tie off the strings. After you've made several such strategic cuts, you'll find that you'll be able to achieve the tautness in the string that you want.
You'll find that hedge trimming entails a little pruning, followed by a little trimming (shearing) -- you'll have to play it by ear.
In Step 4, we'll get those first stakes in the ground....