Sorry, exchange details.
Most commonly it's done by the legal advisor for buyer and seller over the phone (Law Society Formula B).
Each has a signed contract and the buyer's will have deposit funds.
The advisers will go through the contract , insert the completion date and agree the details then they will agree a time for the exchange which is then written on the contract with the fact it was Formula B and the name of each of the legal advisor.
The signed contracts are then sent to each other and buyer's adviser sends the deposit, usually by cheque or, if not enough time for it to be sent and cleared, held to the seller's adviser's order pending formal completion.
Please note that once you exchange you are legally bound to complete on the completion date agreed or there will be penalties under the contract if the other party is ready to complete.
There are so many "warnings" I could give about exchange but don't want to go into it all incase you want to know.
The main things I will say is make sure you are aware of the contract time, penalties, chain etc... ask your legal adviser.
Mortgage funds are a bit of an issue at the moment, if you need a mortgage, if possible I'd try and exchange and complete on the same day if feasible. Difficult if you have a related sale though.
Does that make sense?