Step 0: Make sure you have the PvZ2 Reflourished mod downloaded. If you do not have the Reflourished mod downloaded, refer to this tutorial:

Step 1: Download and extract the .zip file. 

Step 2: Go to CDN.9.4 (Android -> data -> com.ea.game.pvz2_rfl -> files -> No_Backup -> CDN.9.4).

Step 3: Go to a folder in CDN.9.4 named "LEVELS" or "levels". If the folder does not exist, you may need to make it in that directory.

Step 4: Move the level files into this directory.

Step 5: Go to the level that you named the file after. EXAMPLE: egypt1 = Ancient Egypt Day 1 , cowboy13 = Wild West Day 13

Step 6: Enjoy the level! If you wish to allow the previous level to return, you will need to delete the archived level/custom level from the CDN.9.4/levels directory.