You will first have to catch/ defeat registeel, regirock and regice as well as the 3 legendary dogs or 3 legendary birds (depending on which version of the game you have).
First head to the underground and dig for 1 mysterious shard L or 3 mysterious shard S.
Select Yes to digg the wall.
The Wall collapsed.
You will get a Yellow shard and a Mysterious shard L.
Now you need to go to Ramanas park through route 221.
Click the Town Map option.
Enter this building at Ramanas park!
Select trade to exchange the mysterious shards with genome slate.
Select Yes to trade one Mysterious shard L for 1 Genome Slate.
Now head to the Genome room.
Enter into the Genome room.
Select Yes to use a Genome Slate.
Place the slate on the pedestal to fight mewtwo!
You are in Strange Space!
Select Save your progress when asked would you like to save your adventure so far?