It's now time to build the actual website and this stage comes with its own set of challenges. Cut corners here at your peril.
Contract - if you are a developer, ensure that you're using an agreement that is neither too heavy nor too light. If the contract is too detailed, it might cause delays to the project and create distrust. If it is too light, you're guaranteed to run into trouble. I am currently using a modified version of this plain english agreement. If you are the client, make sure you understand points such as whether or not the site will work in all browsers, how many revisions you are entitled to and what might constitute additional work.
Deposit - it's amazing how the mood can change when the discussion of money is raised. I have seen a lot of clients get excited about projects, agree to the budget but then take forever to pay the deposit or stop taking phone calls. Developers - don't start anything until something hits your bank account. Personally I like at least a third but 100% isn't unreasonable if you're going to finish within a week. Some developers refuse to launch a site until the balance is paid which I think is a bit mean. If you have control of the hosting, you might prefer to specify that the balance needs to be paid on time, otherwise the site is turned off. Clients - you'll have a much more co-operative developer if you pay on time.
Communicate - projects need to flow, otherwise frustration sets in from one or both sides. Email is convenient but the phone is your best friend. Clients - we're all busy. But please, only commit to a website if you can make the time to develop your content. Even a small site requires 10 - 20 hours of time to write copy and gather or take photos.
Build - if you have a good web plan and all the content is ready, then its time to Build a Joomla Website. This process is actually one of the easier parts of a project. Nailing down good specifications and content takes discipline but it makes the development stage so much easier.