First Read this. Because this is important for you to understand basics.
https://steemit.com/utopian-io/@jeevanjot/steemoc-steem-overclock-tut-1-get-steemit-hardfork-version-for-beginners

• Now inside Script tags, Put this code

//Where wif is your active private key in account
//username is your steemit account name.
//witness is the name of witness
var wif = "5xxxxxxxxxxxxx"; //Put the write key here inside quotes .
var username="jeevanjot"
var witness = "blocktrades"//Put any witness name over here like "gtg" "timcliff" etc with quotes.

steem.broadcast.accountWitnessVote(wif, username, witness, true, function(err, result) {
  console.log(err, result);
});

Here is demo pic.

https://preview.ibb.co/kLT7Ve/2018_08_05_2.png

Don't show your private keys to anyone!

Now you have done Press RED PLAY button to do the voting from your account.
That's it