It is common to want to contact strangers on the internet for help. Often these strangers happen to be successful, busy people, or even celebrities of some sort. It is not ideal but, without any warm introductions, you have to go for the long shot of a cold email.
The ability to get help from people out of your natural social network is very powerful. It may also be a thin line between a respectful request and an intrusive nuisance. Before sending an email to a stranger, make sure you know it is ok to do it.
It is ok when...
...the person has a public way of contact, like a public email, a form on their website, or open DM on Twitter.
...the contact is personal and specific. If you are contacting dozens of successful people, without any criteria, just to pitch your project, it is spamming, not contact.
...the interest in that person's work or opinion is genuine. It is dishonest to ask for help someone that you wouldn't gladly help if you had the chance.
...you are 100% ok with being ignored without thinking bad of that person. There are a lot of reasons for someone you don't know to ignore you, assume they had a legit one.
If you are not abusing anyone's goodwill, nor transpassing any politeness line, then you can think about the message itself.
The message should be...
...short. It will give you better chances of being read and also demonstrate you won't be a burden to talk to if they chose to do it.
...contextualized. It should clearly state three things: who you are, what they said or did that made you think they could help you and what help do you need.
...actionable. If you are asking for help, tell them how that help looks like. A feedback by email, a meeting, an introduction to someone else, an interview, an invitation for an event. Be very precise about what accepting your request would demand from them, like how long a meeting would take, how long is the material you want them to read, where is the event. Any uncertainty will be filled by the worst case scenario they can think of, so they will just politely decline.
...empathetic. Show that is totally ok for them to decline your request. Of course it is ok! They know it, you know it, but it is nice to put it in words. Acknowledge that they are busy and that you know it is a long shot for you to even get an answer.