How do you get the master sword with 12 hearts?

There is no way around the 13 heart restriction to get the Master Sword; unless you hack the game that is. If you wish to gain the Master Sword, you have two options:

• Locate 4 more shrines and gain 1 heart. If you use the Sheikah Slate or online guide, locating these shrines will be easy
• Complete the The Statue's Bargain quest. Completing this quest will allow you to exchange stamina bars for hearts and vice versa

Based on this guide, those who have the Master Sword during 3-heart challenge runs take advantage of the The Statue's Bargain quest. They will do shrines until they reach enough stamina that is equivalent to 10 hearts. They will temporarily exchange their stamina for hearts, get the Master Sword, and then go back to 3 hearts

There is no intended way for you to get the master sword with less than 13 hearts. You have essentially 3 options (2 of which @Wondercricket already mentioned in his answer):

1. Find 4 more shrines. I know finding some shrines can be sometimes difficult and frustrating. If you really don't want to look for them you can use an interactive map such as this one.
2. Find the Horned Statue in Hateno Village. In Hateno Village, there's a kid running around the west entrance telling you about something cool he found. To learn more, look here.
3. Utilize the complicated early Master Sword glitch. There is a complicated glitch that allows you to get the Master Sword with as few as 3 hearts. At this point, you shouldn't worry about using this glitch as it is much less work just to look for 4 more shrines. If you do want to learn about this glitch, I recommend watching this tutorial by Limcube.

TL;DR: Just look for 4 more shrines, or use an interactive map.

There is also a glitch you can use with the horned statue in the quest mentioned above that can duplicate heart containers/stamina vessels.

Heart container/stamina vessel duplication glitch