New Married Couples allowance
The new married Couples allowance starts from 6 April 2015.
However before you start spending it, it is only worth £212 per year, and it is only available to married couples and civil partners where both Partners are basic rate taxpayers.
Furthermore it is a transfer of personal allowance so if both of you already use your Personal allowance (from 6 April 2015 £10,600) the transfer is pointless.
So if you have a spouse who does not pay tax or is earning below the personal allowance you can elect to transfer part of their Personal allowance to you. To do this you need to register with HMRC on their website
Were you born before 6 April 1935
If you were born before 6th April 1935 the original Married Couples Allowance (MCA) is still available and it takes precedent over the new allowance because it is worth more to you.
You can still elect before the tax year who receives the allowance and redirect it after the tax year if you have insufficient income to use it.
Finally you claim Married Couples Allowance either via your tax return and or by having the allowance included in your PAYE code.