Before you get too far into this Instructable I should mention that the only piece of equipment you'll need is an iPad. Of course, a lot of people don't have these. If you're one of those folks I'm sorry to say this won't work. FormIt might be coming out in the future for other tablets, though. Either way, I hope you appreciate this project.
Step 1: Download FormIt
Step 2: Get Started (With a Location and a Picture)
The first thing you'll see when you open FormIt is the gallery. This is where you copy, edit, or delete existing projects. Since you don't have any existing projects just click on the plus sign in the upper right of the screen to start a new one.
Once you have a new project started you'll end up in a screen with a big grid stretching off into the distance. Don't worry, we'll make this look a lot better right now. In the bottom right of the screen is a place marker button (it looks like an upside-down teardrop with a hole in the middle). Click on that and select "Set Location" to pick where you live and what image we'll use (this will make more sense in just a second). If you have location services enabled the map should already show where you are at the moment. If not (or if you want to design your garden for somewhere that you aren't at the moment) you can enter an address in the bar at the top of the screen.
Now that you have the map of where you want to design your garden zoom in closer to where you'd like using pinch-to-zoom. Next, click the "Import Satellite Image" button in the top right. This will show you a satellite image of the same location. Zoom to a desired level (I recommend the closest level possible, unless you're designing the garden for a palace) and hit "Finish Importing" at the top right of the screen. It may take a second to load, but you should now have a satellite image overlaying the grid. You can use one finger to rotate the view as well as pinch-to-zoom to get a better view of your garden area.
Step 3: Model Some Buildings
There are two basic ways to make buildings in FormIt. Both ways are accessed by clicking the box and plus symbol all the way in the bottom left corner of the screen (See the video below for a demonstration). The first is to use the existing solids that are included in the program and modify them to fit your needs. To do this pick one of the solid images, such as a cube, and then click where you would like it on the image. Double clicking on a face or edge allows you to drag it to a new location, altering the size and shape of the existing object. You can also rotate objects by holding down while clicking them with one finger. You then select the circular arrow and drag a corner of the object to rotate it in place. Dragging an object without selecting any options will move it around.
The other way to create or change buildings is by using the line or rectangle tool. If your buildings are odd shapes this may be the easiest way to create them. Simply draw where you would like the edges of the building to be, and when you complete the shape pull it up to make the building 3D.
This video walks you through the modeling methods in FormIt listed above (as well as a bunch more) so you can start recreating your own surroundings for this Instructable:
When you have finished modeling the buildings that might cast shadows on your garden it is time to move onto the next step.
Step 4: See How the Shadows Move
Here is a quick video on how to utilize the shadow tools in FormIt.
In the pictures you can see how the shadows move over the course of the day by my house (the one in the middle). From this we can see that the front yard gets a ton of light, while the side of the house gets mostly midday sun, and just behind the back of the house gets very little direct sun. Knowing this, I could plant some vegetables in the front of my house, some plants that do well with midday sun on the side, and some ferns or other shade-tolerant plants in the back. You should be able to use the same steps to figure out what to grow where you live!