Doesn't seem too hard. Run some really fat, heavy, knobby tires and keep them underinflated. Then add some lead shot to any bags, panniers, extra water bottles, etc. that you can put on your bike.

But I think you'll find it to be counterproductive. I find that I get home most exhausted when the cycling conditions were most ideal - nice temperature, tailwind, hitting the lights right, etc. That motivates me to put in even more effort to take advantage of the situation. OTOH, bad conditions, like a stiff headwind, just make the ride a struggle where extra effort seems wasted so I just put out as much as needed to keep going and don't get much of a workout.