Not sure why someone else hasn't responded. I'm new but have played something similar called 'The Great Game', doesn't appear to be running now though.
In brief...
You set up a portfolio, it is automatically set up with £100K to trade with.
You then decide what stocks to 'buy' and 'buy' them. For each stock there is a purchase (ask) and sell (bid) price. When you purchase stock, it will be at the 'buy' price. You decide on how many shares to buy and the system will calculate the cost of the shares, the commission and trade costs. For say £10,000 of shares you would pay £10,000 for the shares, £10 for the trade and £15 for the commission, so a total of £10,025. If the share price doesn't change, you have already made a loss of £25.
When you sell the shares, you sell at the selling price, then pay a £10 trade fee (and possibly another 0.15% commission).
So to make a profit, the share price must increase to cover the purchase and selling costs before you make an actual profit.
The smaller the amount you purchase the more the stock has to increase to make an actual profit.
If you purchase 100 shares @ £1, that will be £100 of shares, transaction cost of £10 and commission of 15p. So total cost of £110.15. To sell and break even the share price for selling (bid) must be £1.21, so the sell would give £121, take the £10 transaction cost (and possibly the commission of 18p) so after costs you would have £110.82. An actual profit of 67p and that was for a share price increase of 21%.
If you had purchased 10,000 shares @ £1, the total cost would be £10,000, plus £10 trading cost and £15 commission. Total cost is £10,025. To sell and break even the share price needs to be £1.005 (an increase of 0.5% - a big difference to the 21% for the smaller purchase).
So to minimise the trading costs, do big(ger) purchases, but balance that against the possible loss should the share price go down.
I didn't say it would be a short answer.