Shops, credit lists, main menus, info UIs, button designs, code redeems, and setting UIs are all great to put on your portfolio, as lots of games use things like shop systems and main menus.
Save your design in a folder, then in studio, insert a ScreenGUI and an image label inside of it. For the image, import your design as the image. Anything interactive like buttons in your design will need to be exported and imported separately with image buttons.
Hey! UI is artwork, so you have to treat it that way. I spend the majority of the time sitting down researching to see what styles match the theme of the game, and a layout that makes sense.
The rest is up to you and for you to decide the time you spend on improving yourself. It takes a lot of practice but it’s worth the time.