Buy an equal number of pie shells, strawberries, watermelons and cooking apples from the Grand Exchange (each has a buying limit of 1,000 per four hours). Withdraw 14 pie shells and 14 strawberries, and use one on the other. This will bring up a menu to make part summer pies, so click to make-all. Once you've made 14, bank them and repeat. When all of your strawberries and pie shells are gone, use the watermelons on the part summer pies in the same fashion, and then use the cooking apples on the part summer pie 2s to create raw summer pies. Adding an ingredient 14 times will take about 20 seconds including banking, so you can make up to 840 raw summer pies per hour.

You can then sell the pies on the Grand Exchange. To maximize your profit, buy one raw summer pie at the instant price, and then sell all of yours for slightly below that price.