You will need to install Windows 7 on the SSD while it is in the laptop. If you install Windows while the drive is in another system, you will not be able to boot off of it once you transfer it to the laptop.
1. Physically install drive to laptop
2. Insert your Windows disc/pen drive, and boot off it (disable Secure Boot in BIOS)
2a. If your laptop came with Windows 8 from factory and has UEFI, you will need to disable UEFI boot (or enable CSM) first. Remember to disable Secure Boot as well before trying to install Win7.
2b. If the laptop came with Windows 8 from factory, and you disable UEFI/enable CSM, you may need to use the diskpart command line tools to convert the SSD to a MBR disk.
3. Once you have everything set up properly before installing, you can run your Windows installation.
It may all seem a little daunting or tedious, but you can find plenty of guides online and users hopefully will answer any questions you have here, too.