The images below show how.
The Trello desktop is organized into ’Team’ areas. You start off with MY BOARDS for your personal projects. Your firm would have its own team area that would contain all your projects. A team area makes it simple to provide access to the projects that you set up. I use ALL WORK for the name of my project team area.
Projects are represented by the icons in the firm's team area as seen above. Each project can have one of several background colors to make it stand out.
Most of the time you will create a new project by copying an existing project or a master project. This approach jumpstarts the process. Next add or archive as needed to customize your new project.
The Phases of your project are represented by the lists. Lists could be used for other project information that doesn't fit nicely into one phase or another. These additional lists might be things like 'Project Parameters' where you keep general project information, or 'Special Issues' where you can focus on something critical to your client.
The main Tasks of each Phase are represented by the cards in the lists. Each card contains much more information than is visible while in the list (minimized).
Clicking on the card enlarges it, making several features visible/editable.
The Status of the main Tasks can be shown by using the labels. Or the labels can be a sign of priority or desired focus. This is my suggested meaning for each color.
Use red to also mean ’complete’ or add colors to those standard six. These are suggestions. Anything that works for you is good. With small teams of one or two, labels may be overkill.
 The Task Name
 The Subscription toggle button. Subscribing to a card means you will be alerted to updates.
When you have reference info, research, or a standard that applies to a task, make it an attachment. PDFs and JPGs are most convenient because the open in right in Trello.
SubTasks in the form of checklists are REALLY useful. There are so many things to do in the typical project that using just cards would become very unwieldy. You can have as many checklists per task as you want. Shown here are 4 STEPS* for each task.
Set a due date for the task. You can get these due dates to show on a Trello calendar or a shared firm-wide calendar.
You can assign team members to a Task here. It's a way of indicating who is responsible for the task’s completion. Once assigned to tasks, the member can see all his tasks by clicking "CARDS" in the menu displayed when you click your name in the upper right corner. The member will also get notices of any updates to the task.
Take a closer look at the example I have been describing by subscribing to Architekwiki and then following the link in the archive to PM-Steps v1.0, a public board that you can copy and use to start your own master.
I think that Trello is the best Project Management tool that I have ever seen. When you combine Trello with the STEPS* concept, I don't know how you can beat it. Especially since it is so simple and entirely free.
* If you want to explore the STEPS concept, see this post.