It's surprisingly simple.
(Jerusalem)—This we know: One does not need pen and paper to pray to God, nor does one's prayer—if it is in written form—need to be located in any particular place to be heard. Still, each year, thousands of letters addressed to God are delivered to the post office in Jerusalem and inserted into cracks in the Western Wall.
How the letters arrive at their destination makes for an interesting article published recently in The Telegraph UK. According to the article, "all you need to write on the envelope is 'To God, Jerusalem;' the postman is obliged by international law to deliver it...there, post room workers open the envelopes addressed to God and carefully fold up the notes before slipping them between the cracks of the Wailing Wall."
Post office spokesman, Avi Yaniv, is quoted as saying they also receive letters addressed to others, like Jesus and King David, but the Chief Rabbi at the Western Wall permits only those addressed to God to be delivered.
As to the contents of the prayer letters, said Mr. Yaniv: "They ask God for everything. But last month I found a letter from a person who didn't ask for anything. On the contrary, he thanked God for what he had. He had a good wife, a good house and a good job."