The whole idea of having a PLMA with an oesophageal tube (eg: ProSeal), is to protect the glottis from regurgitation. Any gastric contents refluxing up the oesaphagus SHOULD come out the oesophageal tube. Therefore the oesophageal lumen needs to be lined up with the oesopahgus. This is not always the case when inserting the PLMA blindly. So, to ensure correct placement :

• Lubricate bougie and place in GASTRIC lumen BACKWARDS (Otherwise you won’t be able to remove it)
• Perform gentle laryngoscopy. You only need to be able to see the oesophagus or the back of the oro/hypopharynx
• Insert the bougie gently into the oesophagus
• Remove the laryngoscope and railroad the PLMA into position. There is not enough room to railroad the PLMA with the laryngoscope in the mouth 
• Check adequacy of the PLMA airway as usual (chest movement and ETCO2)
• Remove the bougie and insert a gastric tube to drain the stomach
• You can also use a gastric tube protruding from the gastric lumen, rather than a bougie, but it can be tricky to railroad the LMA over without kinking it

Ref: Brimacombe J. Anesthesiology. 2004 Jan, 100(1), 25-9