It can be a challenging skill to apply, and you need to use your judgement to discern whether the other person is in a place to engage with what you say.
One comment I'd make is the difference between "valid" and "rational". Emotions and feelings are always "valid", in the sense that they are a natural consequence of events and prior conditioning. But feelings are rarely "rational" - they often don't reflect the complete truth of a situation. For example, suppose someone says "Jennifer sent me this short snippy reply today, I swear she's upset with me about something and won't tell me what it is". It is perfectly legitimate to validate that you can see where that fear comes from, but nevertheless offer alternative possibilites: maybe Jennifer is going through a tough time personally, or has a really tight work schedule at the moment. You don't have to fully buy into someone's thoughts and feelings in order to help them process them. In fact this is rarely going to help.
One comment I'd make is the difference between "valid" and "rational". Emotions and feelings are always "valid", in the sense that they are a natural consequence of events and prior conditioning. But feelings are rarely "rational" - they often don't reflect the complete truth of a situation. For example, suppose someone says "Jennifer sent me this short snippy reply today, I swear she's upset with me about something and won't tell me what it is". It is perfectly legitimate to validate that you can see where that fear comes from, but nevertheless offer alternative possibilites: maybe Jennifer is going through a tough time personally, or has a really tight work schedule at the moment. You don't have to fully buy into someone's thoughts and feelings in order to help them process them. In fact this is rarely going to help.