You would keep the same weight for your second set. On your next workout day, you would increase the weight. Let's say 185 was your 15rm for an exercise. You would work backwards by 5-20lbs depending on what you want. We'll use 10lbs for this example. It would look like this:
Workout 1 (Monday) 2 sets of 135
Workout 2 (Wednesday) 2 sets of 145
Workout 3 (Friday) 2 sets of 155
Workout 4 (Monday) 2 sets of 165
Workout 5 (Wednesday) 2 sets of 175 & finally
Workout 6 (Friday) 2 sets of 185
I use 10lb increases for everything except legs. On legs, I use 15lb increases. Hope this helped.