This is a bit of a pet peeve of mine, but recommending pull-ups to someone out of shape who needs to loose weight is just bizarre.
If someone was a beginner on the bench press, would you start them at 250lbs and yell at them to push harder? No. That would be stupid and dangerous.
Yet somehow fat people are supposed to will themselves to do a pull up, which is basically the equivalent.
Yes, a pull-up is a tall order for someone who is overweight or for most females - and even if you can do one or two pullups, that's not a good basis for working out that part of your body. You should be shooting for 10-12 reps, etc.
However, a "pull-up" can really be though of as any compound exercise that requires you to use your lats and biceps to pull yourself up OR pull something down to you.
So look for the "lat pulldown" machine and adjust the weight accordingly. It's the same thing.
I was able to start doing pull-ups after a few months of doing bent over rows with a $25 set of resistance bands.
It is very difficult to figure all of the details out, because there is so much money to be made by convincing people to buy into gimmicks. The OP has the right general ideas, though.
Some trainees would be better off simply stretching for a month before doing any resistance training.
These details need to be assessed on a case by case basis. This is just an overview.
Negative pull-ups or lat pull-down could be used until the required strength is there.