Easiest is to use their API. Create a new subscription with your plan but set first_renewal_date and/or trial_ends_at to the next minute or so. You can also try to update an existing subscription and set timeframe to "now".
API Documentation: http://docs.recurly.com/api/subscriptions
Use one of their client libraries (for PHP, Ruby, Python or .NET): http://docs.recurly.com/client-libraries/net
Don't forget to use your test account so you don't fill up your production account with test subscriptions.