Open an account with one of your service's designated brokers, and then find out from your brokerage how to get set up for autotrading.
There is no set way of doing this and each broker will have it's own requirements so make sure you find out how each one operates
(and don't assume they are all the same because they're not).
Once you're set up, you will be able to nominate not only which advisory service you'll be autotrading but also which
specific trading system of theirs you want to use (if they have several, which many of them do).