The UI is important on every device because it's the first layer of design that directly communicates with the user. If the buttons are confusing and abstract, then your target audience will not know how to navigate, and this can reduce the conversion rate of your site or your application. Here are some fresh UI design tips to help you out.
Clarity in Design
The user will simply ignore those buttons that make no sense to him or are not clear enough. Sometimes just clarity alone does not work -- you also need to make sure the buttons are large enough to be pressed with a finger, or thumb, whichever the case is for you. For computers, you would also need to consider if visibility is high enough to be seen on a large screen.
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Justification in Context
Justification in context is basically justifying why the interface is placed the way it is. For example, you have a pause button on the left top corner of your game -- why is it there? The simple and obvious answer is that one, it's not interfering with the graphics of the game, and 2, it's visible upfront and right at the top for the user to use whenever he needs to quickly pause the game.
This is why it's very important that you clearly understand your audience and then come up with a list of ideas on where each element will go. Each feature has to be justified depending on context.
Having Default Text or Content
It's hardly noticeable, but this little trick will seem like magic. When it comes to UI designing, the usual aesthetic is that you put default text in various field options so that your user knows where and what he needs to input. This will not only save your users a bunch of time, but will also provide you a clear output for analysis. The most common example would be contact forms, or fields that you need filled for sign ups or other applications.
Having said all this, designing a UI is a very personal and opinionated experience. You need to put yourself in the shoes of your users and determine why and how you need to structure the layout of the application or site you are building.