Since Stretch is no longer the latest version of Raspbian, you'll have to look for the exact image you want here for full versions, and here for lite versions. Images are sorted by date, so if you want to latest version of Stretch, click here (full) or here (lite).
Either version (full or lite) will work for this guide, but the version you choose depends on your project.
You'll probably need to use an adapter.
If you're using MacOS, you can use the following instructions. If you're using Windows, scroll down.
To open Disk Utility you can type cmd + space bar and search. Or you can use Finder to navigate to Applications > Utilities > Disk Utility.
You'll need to format the SD card to FAT-32. To do so you'll need to click on the "Apple SDXC Reader" in the left column of Disk Utility.
Click the "Erase" tab.
Choose a name for the card (I chose Jessie). For the format, choose MS-DOS (FAT).
Click "Erase" when you're finished.
Using the df command we're going to determine the mount point.
In the output, under the Mounted on column look for the name of your SD card. I named my JESSIE so I'm looking for /Volumes/JESSIE.
Then take note of the filesystem name in the first column. Mine is /dev/disk2s1. This is actually the partition name, but what we're really concerned with is the device name which is disk2. Copy this device name.
For this step you can use either Disk Utility or Finder. In either case look in the left column for the name of your SD card. In my case it's JESSIE and click the eject symbol.
Hopefully by now the disk image is done downloading. By default, the disk image will download to your Downloads folder. Open up the Terminal application and type:
To locate the img type:
ls -lha | grep jessie
You should see some output like this:
Downloads> ls -lha | grep jessie -rw-r--r--@ 1 tyler staff 292M Feb 25 15:04 2017-01-11-raspbian-jessie-lite.zip
Now unzip the image. Make sure to use the name of your file.
Go back to Terminal (ensuring you are in the same directory as before), and type:
sudo dd bs=1m if=DISK_IMAGE_NAME of=DEVICE_NAME
Make sure to replace the values for DISK_IMAGE_NAME and DEVICE_NAME. Mine looks like this:
sudo dd bs=1m if=2017-01-11-raspbian-jessie-lite.img of=/dev/rdisk2
Enter your password, wait until it finishes, and you're done.
Note: You may have noticed that we're using rdisk2 instead of disk2. You can actually use either, but rdisk2 is significantly faster.
If you're using Windows to format the Disk you can use the following steps.
Open Windows Explorer and find your SD card. Right click on the icon and click Format.... Make sure Fat 32 (default) is selected and hit enter.
Go here and download the app called Win32 Disk Imager. After downloading the program, click to uncompress the files, and then click Win32DiskImager.exe to launch.
In the Win32 Disk Imager, click the blue folder icon and locate the Jessie image you downloaded. Then in the Device box make sure the drive letter for your SD card is selected.
Click the Write button to begin.