To get to San Ignacio de Mini the easiest way is to fly to Puerto Iguazu, or alternatively to Foz do Iguaçu, on the Brasilian side if you are planning to visit also The Iguazu Falls.
From Puerto Iguazu an early morning bus connects Puerto Iguazu with San Ignacio de Mini.
The bus is comfortable and the ticket cheap, the travel lasts little more than 4 hours.
You can plan the visit to San Ignacio de Mini as a day trip and take the return bus around 4 pm to Puerto Iguazu.
The ticket to the ruins of San Ignacio entitles you to access many other Jesuit ruins scattered in the area and certainly worth a visit, although not as impressive as San Ignacio.
If you want to see also the other Jesuit Reductions of the area you should consider a night step over in San Ignacio.
Definetely, if you are visiting San Ignacio de Mini you should also visit the fantastic Iguazu Falls.