It depends how important it is to you.
Ideally you'd do it on off days, and even more ideally on days not immediately following leg day.
If you don't really care all that much about recovery and getting the best out of everything, or your schedule is a mess, then yeah just do some lower intensity/shorter bouts after your reg training (I wouldn't do it before).
Or figure out ways to incorporate "cardio" into you training, like the superset talk that's going on right now.
Finishing a workout with complexes is something that works really well. Just pick 1-2 complexes and do 6-10 total sets, it can be brutal.
I personally lift 5-6 days/week, and only do dedicated, traditional cardio on the off days. But I do 15 mins of jump rope with a weight vest after my 2 leg day (my calves suck) and as long as you're relatively proficient you can make it pretty tough and it's kind of fun. Then 1-2 days a week I do something else at the end, sometimes a complex relating to that day (whether its push, pull, legs...), sometimes regular jump rope, sometimes sandbag work and various loaded carries.
Basically, there's a lot of different ways to go about it, it just comes down to your own time constraints, preferences and fitness goals.
If it's just for general fitness (ie/ feel better and not like a fat shit walking up stairs) then I'd probably do 10 mins or so at the end of a session and then something bigger on off days.