Good luck!
Rest time for you will be two minutes: any shorter and your circulation/stress factors won't have reduced to a 'normal' level (i.e. you'll still be "tired" from the last set).
For the sake of form, I would recommend having a bit of prep before hitting the rack. Lie flat on your back with your heels tucked up under your bum. Now, arrange your lumbar so that you can get one of your sets of fingers under your lumbar spine (i.e. a gap of around a finger's wide between the floor and your spine) Now squeeze your 'abz' as if you're fighting a battler. This is 'core' bracing in a neutral position. Your spinal position shouldn't change, but should solidify and be nice and robust.
Now, before slacking off this bracing, push both your heels into the ground for a 'glute bridge' or whatever it's called. Keep your bracing the entire movement and lower yourself back down - fully braced. Now feel the gap between your lumbar and the floor: still the same? Then you're good to go. If it's different, relax, and then - still lying down - tilt your pelvis around (this might involve some weird/wonderful muscles contracting, but it works...) to find that "neutral" position.
If you can hold this position through the glute bridge, you can hold it when squatting. I would recommend starting with goblet squats first as these are the most forgiving: keep the weight over your heels and sit down between your ankles.
I would strongly suggest taking a video, too, as the chances are you won't get down very low (being office-based) and will have very short Rectus Femoris muscles, as well as very tight hip adductors/extensors. As you descend through the movement, watch for where your lumbar starts to tuck under, following your bum: stop your descent and this point and go back up. And then visit the Mobility thread (the link is in the OP of the GymRats thread). This will stop your lower back hurting if your mobility isn't great.
Warming up? This is pretty much everything you feel comfortable with to get comfortable with the movement. you're about to perform. For squats, I generally perform the 'squat' without a bar (hands overhead or out front) for around three sets of ten reps to make sure the muscles concerned are awake and not too stiff. But it varies from person to person... some will attack the bar straight off; others need to row/cycle for five/ten minutes before even thinking about lifting...
If you think of it as preparing your central nervous system for activity, then you're 80% the way there.
Finally, regarding the rows, use exactly the same principle as for the squats: brace your core so that the load transfers from your back onto your hamstrings and glutes. Remember: your back problems were probably caused by poor core strength and even worse mobility (at the hips, between your vertebrae and probably even your shoulders and pecs), so take this slowly. You should be able to feel if your back is completely flat (first off), but if you're doing it right, your hamstrings will start to ache pretty soon.
And guess how my back problems were cured? Lifting weights. Because my back is now used to being in a neutral position, and can be held there very effectively. My physio explained I had the horrific back pain simply because I was a pathetically weak person, so getting strong = getting better.
Regardless, we're here to help, so any questions: fire away!
Good luck.
(j/k have wanted to do that for ages...)
