Those already have the trading account will say, just call broker and say the num of shares of company to buy or sell.
The next question will be, how to open a trading account? Its simple, just need to make a phone call to the stock agency and tell them you wanted to opening a trading accpunt to start trading shares. They will make sure u have all account needed for the trade.
Below are some of the agency in Singapore:
2) Lim and Tan
3) UOB Kay Hian