In addition to what
@Bra****
has said, when you're on a 12-month Retirement extension you just have to obtain a re-entry permit before leaving Thailand (you can buy these at the airport), if you're using proof of pension you need to get your pension payment certified by way of affidavit at your embassy in Thailand. This must show minimum of equivalent
*****
baht per month. Generally you will need to show a 12-month lease for where you are living (it doesn't matter whether you have just started the lease, or are halfway through it, as long as it is 12 months). I say "generally" as each immigration office make their own rules on this, so best to check there first.