So here is a guide to what is the etiquette when you see a person that you find attractive.
We shall assume this person is a stranger to you, they may even be in a public place. It is fine to find someone you see as attractive, it is fine to fancy them, however here are a few things that you should not do as a consequence of you fancying them.
Do not assume they fancy you. This is very important, you should have learned this at about 3 years of age, things you experience are not necessarily experienced by others.
Remember, you are not obliged to do anything. It is perfectly acceptable to see an attractive person and then go about your day. Seeing them, like seeing great art, might cheer you up and make the day better. This is a major benefit of seeing someone you find attractive.
Do not yell "Oy Oy!!" or "Blimey love I could give you one!" or "Sexy lady, give us a smile!" or "Hey stud, I like what's in your jeans!" or "I would ride that beard like a rodeo bull" or any similar comments. In fact do not yell at someone, it's pretty tacky at best and can be frightening at worst. Just don't.
If you are wearing an amusing T shirt suggesting you are a 'sex instructor', 'tit squeezer', or 'beard fondler' you may want to wait until another day, but certainly do not reference the T shirt as a witty chat up. Chances are they will not find it witty.
Do not whistle at them, some ladies do like a whistle but the vast majority do not, I don't know about men but I would err on the side of caution, don't whistle. People are not dogs to be summoned.
If you decide speak to this attractive person do so politely, maybe try "Hello" or "It's a lovely day" (unless it's raining then you will look like an idiot, if you are going with weather related chat be sure to know what the actual weather is like). You could be more direct and launch straight into "Sorry to bother you but you are very pretty/handsome and I had to say hello" but it's a riskier approach. Always smile, but not like a serial killer if you can help it.(I do not know what a serial killer smile looks like...but you get the idea)
If the person looks scared, or ignores you. Apologise and walk away.
Do not block their path so they have to stop, do not demand they speak to you, do not badger them to be nice, do not tell them to cheer up, do not insult them or call them frigid. Be nice.
If they smile and chat back, do not assume 'you are in' and suddenly revert to telling them you fancy them rotten and do they want a shag. Keep being nice. Maybe arrange to have a coffee, or offer them your mobile number so you could stay in touch.
If they chat back but make it clear they are already in a relationship or not interested in a new one, see advice re what to do if they ignore you above.
Well done! You have politely spoken to a person you fancy. You have not scared them or been rude. Who knows what will happen next...
If you have any tips do add them in the comments below.