I got this and Instant Offense at the same time.
Start a tip-off game, set it to rookie, and choose a team with a shooter. I chose the Mavericks for this one as I've found that Calderon makes it rain threes, even more so than Steph Curry or other prominent three point shooters.
Quarter length can be whatever it wants, I got this with regular 5 minute quarters. Turn off auto subs and set your shooter (in my case Calderon) to the bench. Then when the game starts, call a time out and put your shooter in the game.
Then just start shooting threes. Using LT to help set screens will occasionally help you create space, but if you use a guy like Calderon I've noticed he hits them pretty well even with someone in his face.
Make it Rain will pop once you finish the game assuming you have made 10 three pointers and are above 40%. Instant Offense will pop the moment the player gets to 20 points.