> How do I tell them ...

That is very simple. Just write an e-mail and tell them whatever you need to tell. Just be polite and respectful.

WARNING:  this will not at all guarantee that they will wait 6 months for you to make a decision.

---
You probably wanted to ask something like:

> How do I negotiate to start a job not now, but after 6 months?

That is already another thing. You do not ask a question, but you need to start a negotiation. Be ready to have answers and arguments for everything that you say and for everything that you might be asked. However, 6 months delay without a very strong argument is unlikely to happen.

---
Considering that you accepted another job while this company was still deciding, you might just tell them that you need to turn down their offer because of the new job, but you really want to work for them in the future. Be ready to tell them when they should contact you again for another job in their company.