We were supplied with an SQL Server STD 2012 license and the supplier told us that to downgrade to SQL 2008 R2 Enterprise we have to buy this license first and then downgrade. Does anyone know how we can do this? I never installed SQL before.
It's true that to install 2008R2 now, you have to buy a current licence (i.e. SQL2012) and use the Cross Edition Rights to downgrade, however a SQL2012 Standard licence will only allow you to use SQL2008R2 Standard and below, not Enterprise edition. For more details check out the SQL Server 2012 Licensing Quick Reference Guide (Google that and it will come near the top - it's a PDF) For complete certainty, speak to your licensing reseller.
I agree with Kev Riley, all my talks with MS Licensing and working with virtual servers, EAP's, EA's, etc have stated that SQL 2012 license allows you to run 1 version downgrade. Purchase version 11 and you can run 10.5 of the same edition.