Flip Merchanting is proberly the most simple form of Merchanting you don't need much money to start out for this method but about 1m+ is reccomended.
Choosing the item: This is proberly the hardest part of Flip Merchanting because if you choose the wrong item you could lose money. The best items to flip are items which have a large demand and that are easy to obtain in the game, e.g. Raw Monkfish, Yew Logs etc.
Also the item needs to be faily stable (only going up or down a few gp). If the item is crashing when you buy, you won't be able to sell, and if the item is rising a lot, it will be difficult to buy.
Buying the item: This is fairly easy, if the item is rising, then buy the item(s) for a few gp over market price. If the item is falling, buy the item(s) a few gp over minum price. If the item(s) do not buy instantly, be patient and keep your offers in the ge, if you wait and wait and wait and nothing is buying, put the price up a few more gp. If still nothing buys try another item.
Selling the item: This is also easy, if the item was rising then sell the item(s) for a few gp under maxium price and be patient. If they don't sell drop the price by a few more gp. If the item was falling then sell a few gp under market price, again if they don't sell drop the price by a bit more.
Repeat this method over and over again as many times a day as possible and if you can afford to flip multiple items at a time (as long as they are from different category's in the four hour limit).
If you have good capital it is possible to make over 10m a day from this method.