Two excellent videos to share with you here. They show you how to stake / nominate your dots (or KSM) via the Polkadot.js UI.
One from Bill Laboon at W3F and one from Sam Eugene a community member. The first one from bill is essentially an official video althogh the UI has updated since he did the video back in June.
Sam uses the new UI and also dives a bit into how he thinks about selecting which validators to nominate. One worth checking out: