1.) Find a Movistar store (One of the Spanish networks). The
Store locator is not too helpful but gives a rough orientation. All or most Carrefour stores seem to have a Movistar store. But in all their three lacations of Palma de Mallorca the Tuenti SIMs were sold out when I tried.
2.) You need your ID and EUR 12 and an E-mail address. The shop assistant will ask for your Spanish permanent or semipermanent address but you do not need one to buy a Spanish SIM.
3.) Note: Tuenti has three different sizes of SIM cards missing the the new one with the micro SIM to break out from the standard SIM. So be sure to get the size your phone uses. In the Movistar where I bought my SIM the two smaller sizes were sold out.
4.) After the registration process (in the shop) the connection is active immediately. You get confirmation text messages with your phone number, the chosen tariff and a link to register in tuenti.com.
5.) When you chose the 6 Euro (+ VAT) package (1 GByte, voice calls: 3 cent per minute) the remaining credit is 4.74 Euro.
6.) All managing is done by web access except the usual GSM codes to activate and deactivate voicemail inbox. If you use the card for short term holidays do not forget to deactivate "auto renewal". To keep the connection in function you need to load EUR 5 every 6 months. International roaming is activated by default.
7.) Callback calls work.