A whole app is a big place to begin. Pick one person and one thing they are trying to do. Then design only the steps that help them finish it.
Make one small flow easier to use
Start with one task, watch someone try it, and let that feedback guide the next version.
Start with the moment that matters
Maybe someone needs to find an event, create an account, or submit a project. Write down what they need to know before they start and what a successful finish looks like. This gives every screen a reason to exist.
Build the flow in small pieces
Sketch the button, then the form, then the screen around it. Use plain language for labels and show what happens after someone takes an action. Make the next step easy to spot, and make mistakes easy to recover from.
Watch someone use it
Ask a person who has not seen the design to complete the task. Stay quiet while they try. Notice where they pause, what they expect a control to do, and what they miss. Fix the biggest point of confusion and try again.
Small, tested improvements add up to a much stronger product. You do not need perfect screens before you learn whether the flow makes sense.