Cardiovascular exercises: use a heartbeat monitor and check that the frequency is within a certain range during the exercise (you can look up how to find the range on the internet). This way, you will not be pushing yourself over your limit.
Weight lifting exercises: choose a weight such that you can do the exercise 12 times (reps) correctly without getting exhausted/fatigued. Then correctly do 4-5 sets of that exercise, with 1-2 minutes between the sets. Optionally increase the weight a little between sets, and reduce the number of reps so that you can do at least 8 reps in the last set.
My personal cardio "training" routine was to "run three times", which means, I would run as long as I could (which may not be very long at all!), then walk until I was recovered, and then run again, until I'd run three times. If I'm running outside, I'll run "out" in one direction until my three times are up, and then make my way back at whatever pace I can. When I'm running on a treadmill, I'll usually stop after my third recovery period, but I'll increase my speed throughout the run.
I don't over do it with this routine. If I'm out of breath or something starts to hurt I just stop running and walk for a bit, but it helped me work up to a reasonable amount of cardiovascular stamina the couple of times I've fallen completely out of shape. I usually abandon it once I have enough stamina that I become time-constrained on how long I can run rather than physically.
Weight lifting exercises: choose a weight such that you can do the exercise 12 times (reps) correctly without getting exhausted/fatigued. Then correctly do 4-5 sets of that exercise, with 1-2 minutes between the sets. Optionally increase the weight a little between sets, and reduce the number of reps so that you can do at least 8 reps in the last set.