This happens at the same time as step 13, but I thought I'd make another step of it to make sure you notice it.
When you sign your contracts you will also need to pay the rest of your deposit. This is standard at 10%, but it is usually possible to negotiate it down - for example if you get a 92 or 95% mortgage you may only want to pay 5 or 8% as the deposit and have the rest come directly from your mortgage. Once you have a valid reason your solicitor should be able to agree this with the vendors solicitor.
As this is generally delivered as a bank draft (not a cheque), it is a good idea to arrange the bank draft before you sign (a day or two should be enough time, but you should confirm this with your bank as they differ).