With a hedge trimmer.

If the branches are too thick for the hedge trimmer you'll need to work your way through it with either long handled loppers or a saw, as suggested above.

to keep the shape get a spirit level and put up a couple of short level goal posts out of cheap woo and run the trimmers along it . you could use string but you mite cut threw it and thewood you can rest your trimmers on making it very easy .

An easy way , if you have a chain saw, is to just hold it at the level you want and walk along the hedge cutting as you go, it will look a bit of a mess at first, but within 6 weeks it will have greened up again, then you just maintain it as usual. derekm

Though with branches as thick as those are likely to be, I would cut at 3', or 3' 6", so the branch stubs don't keep showing every time you do your regular trimming in the future.