Yes, keep the cable connected to the speedometer. Jack up the car from either under the front cross member, or the rear axle/ differential.
Make sure you place jackstands under the corners of the frame...otherwise don't crawl underneath the car without it being supported. I've heard too many grizzly stories of either the jack giving out, or cheap jackstands collapsing and people getting killed or seriously injured.
I can't stress safety enough.
DO NOT....I repeat, DO NOT jack the car up by the bumpers. If you're jacking up the front only, it wouldn't hurt to place bricks or chocks under the rear wheels...just the opposite if you jack up the rear.
The parking brake will only work on the rear wheels.