A bowl of water, a dish of salt, a white candle, a quartz crystal, a stick of incense (sandalwood is very spiritual, if it comes with myrrh that's even nicer) and a piece of fruit as an offering.
Place the water bowl in the (East, southern hemsphere, West, northern hemisphere), the incense in the (West, southern hemsphere, East northern hemisphere), the dish of salt (South, southern hemsphere, North, northern hemisphere), the candle in the (North, southern hemsphere, South, northern hemisphere), the fruit and crystal in the centre.
(This is an outline for what to do):
Thank the God and Goddess, the Universe, Divinity, Deities, the All That Is, all the angles which have helped you and anything else you feel the need to thank like a passed on soul or a spirit guide perhaps.
Tell Them that you have left some food for them and wish to thank them.
Say blessed be and if you can leave all (but the fruit, throw that into a tree) outside for atleast overnight, but if you can't leave it out for that long take it back inside.
It could remain a personal thank you altar for the angels.