Around half of all internet traffic globally is on mobile devices. However, we tend to design websites on desktops or laptops and then see if they work on phones as an afterthought. If you already have a website, make sure you have Google Analytics so that you can see how many people are coming to your website on their phones. Or, go back to your customer profiles and think about if they are likely to be browsing on their phone, tablet or on a laptop? If you know people are going to be using your site primarily on their phones then plan it as such: make a site that works for mobile and then scale it up for desktop, not the other way around.
Remember viewing areas are smaller, attention spans are shorter, fingers are fatter than a mouse cursor; so spend a lot of time reviewing your site on your phone and making sure the user experience is just as good. If you have an e-commerce site, go sit down with your phone and place a test order. Check that everything works, that it’s obvious, and (if you need to) change the layout for a better mobile experience.

