you have the be the same or close to the same soul level, soul memory, and in the same Ng. Boss souls, any covenant item(sun tokens example), eagle feather,keys, and reusable items cant be traded.
Part2, to trade, either drop a summon sign, it can be either shade,red phantom, dragon phantom, white phantom signs. Any sign that you can drop and be summon can be used to get into another persons world to trade.
Part3; you select "LEAVE" to leave the item on the ground, will show up as a small brown bag(similar to trading items with the birds in "Things Betwixt")