How to practise, ear piano playing after years of sightreading only?

Here are some suggestions, have a try and let us know how it goes:

Get a feel for the tune

• CD player / Media player, play a tune that is not too complicated you have never learnt
• Listen to the whole tune a few times until you can hear in your head what the next melody will be before it plays

Learn in parts - Play a few seconds at a time (a phrase) and just find the root note by ear on the piano

• Build up the chord until it sounds most similar to the tune
• Play the next phrase on the CD / Media player and do this again

Test your playing

• When you think you have got a section to a good standard, listen to the whole thing on the CD / Media player to see if you have any differences and correct them, repeat this
• Sometimes you will struggle and need to check the sheet music to see if you are correct, if it sounds right and the sheet music is different don't assume you are wrong, the sheet music may be wrong or your piano may just sound better playing the chord in a different arrangement.

Eventually after doing the above for a lot of songs you will find you may not even need to hear the song, you can just do it from memory of what you think the song sounds like. But this takes time and practice.