Once you've found a prospective tenant you need to work out if they can actually afford the rent, otherwise you're wasting your time and theis by referencing them. To get an approximate gauge of affordability take the monthly rent amount and multiply it by 30. This amount (or more) will be what the tenant needs to earn in order to be sure they will pass the affordability check. The amount can be split by the number of prospective tenants. For example a rent of £1000 per month requires earning of at least £30,000 per annum. So if there were 2 tenants, one of whom earned £20k with the other earning £12k their total earnings would be £32k, exceeding the required threshold of £30k.
If tenants fall short of the earnings threshold but you still like them then ask them if they can find a suitable guarantor.
To carry out a reference the property will need to be create in your account. That doesn't mean you need to pay for an advert listing, you just need to create the property if it's not already there. If you don't want to list it for rent, go through the creation process but don't pay. Click the 'services' link from the top menu and select 'tenant references', then follow the instructions from there on.