WIth Mac's native software and a camera-equipped laptop, you have everything you need to take your own passport photo.
Prepare yourself and clear an area of wall for your background. The Gov.uk guide states that photos should be taken against a cream or light grey coloured background. If you have nothing suitable in your home, you could buy a poster-board to tack up temporarily.
Open Photo Booth (Macintosh HD/Applications/Photo Booth). Depending on your operating system, the application might immediately go into full screen, but in any case, your camera will be activated automatically. You can exit full screen by pressing "Esc."
Hold or prop the computer in front of you to set up your shot. Press the red button. You will hear a series of three beeps before the photo is taken. Take as many photos as you like until you're satisfied with one.
Locate the photo you like best in the filmstrip at the bottom of your screen and press "Control" and click the photo to see "Reveal in Finder." Select that option. (If you are planning on using an online passport photo service, skip the following steps. Just make sure you save your image in a way you can easily find it).
Drag the image from Finder to Preview, or if Preview isn't in your dock, hold "Control" and click the image, then navigate to "Open With" and select "Preview."
Navigate to "File" in your Preview toolbar and select "Duplicate." Doing this will ensure you still have the original image in case you need to execute the following steps again.
Hold and drag your mouse over the image to define the edges for cropping. Use the UK.gov guide to orient your face in the frame. It's better to leave a little too much space at the sides of the image than not enough. You can crop again later, if needed.
Navigate to "Tools" in your Preview toolbar and mouse down to "Crop" (or press "Command" and "K"). The image will now be cropped.
Navigate to "Tools" in your Preview toolbar again, but this time, select "Adjust Size."
Use the drop-down to the right of the numbers you see and select "cm." Your photo needs to be 3.5 cm wide and 4.5 cm tall. Adjust the height first. Having both width and height exactly correct isn't the most important thing because you will have to cut the image with scissors later. If you're in the general ballpark with the width, and have the exact height, your photo is close enough to the correct size to use as long as you don't go under 3.5 cm in width.
Change the resolution to at least 180 (this is pixels per inch) before exiting the size adjustment menu.
Go to "File" then "Save" in your Preview toolbar. (You will only see this option in more recent operating systems if you duplicated the file at the start of this project). Save the file with whatever name you like, wherever you like. You are now prepared to email it off for processing or save it to a pen drive for processing. Be sure to tell your image processor that it's for a passport photo and needs to be the exact size, as saved.
You might skip the photo cropping section of this guide if you use an online service to process your photos virtually. Check into services like Photobox, which will send your photos to you after you process them online.
You can also use Makepassportphoto.com to crop your photo online and make multiple copies on the same sheet to download and take for processing yourself for free.