First of all, You need to know that all version of TheMyth can use this installation guide.
Secondly, Your Galaxy Ace need to be on Gingerbread 2.3.* before you flash TheMyth
We have 7 steps to install TheMyth on your Galaxy Ace:
Step 1: Download TheMyth Roms - Choose 1 version of TheMyth in Download Page
Step 2: Install CWM recovery - This is necessary tool to flash TheMyth Rom. Download it then put in your sdcard. After that, open recovery mode (Home+Power) , choose update from zip and choose CWM setup file to install.
Step 3: Put TheMyth Roms in your sdcard
Step 4: Open CWM recovery (Home + Power), select "Install zip from sdcard" and choose TheMyth Rom then reboot after setup is completed
Step 5: Wait for your phone to boot. After it opens, You can go to menu then open Tweak Manager and enable:
+ VM Swapiness
+ DT: Auto Start
Step 6: After u finish with Tweak manager, continue to open A2SDGUI.
+ Enable Swap and Zipalign.
+ Open 2nd tab and select "sd-ext".
Press Move and wait for your phone to reboot. It will reboot several times so plz wait :)
Step 7: Enjoy TheMyth !