To search by image, tap the three dots on the top right corner of the Google Chrome browser, and then select Desktop site. This will give you the full desktop version of the Google Images website, which includes the camera icon you need to upload or take a photo for your reverse image search. Don't worry, it's easier than it sounds!
Now, tap the camera icon in the search bar, and you'll be given two options: Upload an image or Take a photo. Choose the one that suits your needs - if you've already got a picture of those shoes on your phone, upload it; if you're still out shopping, take a quick snap. Either way, Google will do the rest, searching the internet for matching images and giving you all the info you need.