Meanwhile is normally used at the beginning of the second clause. It comes after a full stop and is always followed by a comma.
If you are in doubt, you can think of the following question: Can I translate the word as Mientras tanto? If so, you need meanwhile. If you can't, you need while.
1. While I was cooking, Alice was reading in the garden.
2. Alice was reading in the garden. Meanwhile, I was cooking.
3. Wait for Peter here. Meanwhile, I'll try to park the car somewhere
else.
4. It rained heavily while we were on the island.
5. One of the suspects was being interrogated, while the other was
waiting outside.
6. One of the suspects was arrested yesterday while he was driving to
the airport, and is being interrogated at the
moment. Meanwhile, the other suspect is still missing.
7. He spent the whole morning ironing. Meanwhile, I was in my room
studying.
