When you’re designing screens, it’s better to start with what already exists. This is not just quicker, but it also means you’re reusing stuff which has often already been proven to work.
The GOV.UK Design System has a collection of patterns and components you can use to very quickly design usable and accessible services.
When conceptualising ideas, you can use the GOV.UK Sketch Kit to mock up screen designs and wire-frames.
When you want to build working prototypes, you can use the GOV.UK Prototyping Kit to test your ideas.
By using these resources you will leverage a lot of best practices and research on accessibility automatically.
For more in-depth accessibility considerations when designing screens you should read the following job role guidance: