The easiest and most reliable thing to do would be to flash stock JVQ with repartition (after backing up of course!) and make sure it's factory reset and the internal SD is clear. You can get JVQ from samfirmware.com and you can also get Odin to flash it with. If you don't know how, post back here and I'll tell you. Then you can just flash
Speedmod for gingerbread to give you CWM back, then simply install the Twooey II zip through CWM and it should do everything for you.
By flashing JVQ you get the Gingerbread bootloaders and a clean base to start from, which I think is important as lots of people report random issues when they haven't done this properly. I've always used a clean base to change ROMs, and never had an issue.
EDIT: I'll try to sum up each step:
1. Backup
2. Factory Reset
3. Format Internal SD card (sometimes called USB storage).
4. Charge battery to full.
5. Flash stock JVQ using Odin with repartition. Reboot.
6. Flash Speedmod for Gingerbread T24 using Odin. Reboot.
7. Put Twooey II zip on internal SD card.
8. Enter recovery mode (CWM) and install Twooey II zip. It shoud clear partitions and reboot on its own, first boot takes a while.
9. Enjoy MIUI and restore what data you need. I recommend SMS Backup and Restore for messages, works well.