Ok this is a guide that follows exactly what I have been doing, and comes with an excel spreadsheet to support. If you don't have excel, you can use libreoffice or openoffice instead.
I am going to go on the assumption, that like I did you are starting with a simple 500k but you can start with as little or as much as you'd like.
Open the spreadsheet, and on the right hand side enter the amount you are starting with.
Then take a look at my example row (2) where I bought oak logs. You will notice that some columns are coloured in BLUE. These are the only columns you should enter data in. The ones in white all have formulas to make this easy for you.
This spreadsheet assumes you will attempt to buy items at 2.5% less than mid-price, hence why it asks you to enter mid-price. The recommended buy price (RBP) is displayed after you enter this. You don't have to follow the RBP it is simply a guide. After choosing an item, fill in it's name, and mid price, then decide on your buy price and quantity and enter those.
What I suggest here as well, is up until the item actually buys, enter the total buy price in the Total Cost column to update your available balance.Once the item has been bought, enter the actual amount you spent in the total cost column. You will notice that RSP was updated after you entered the initial buy price. This is a 5% profit number. Again, it is a guide and you do not have to do it. So now you have your items, sell them back to the GE at your price and update the spreadsheet. One they have all sold, enter the amount you get back in the "Actual Sell" column. This allows the spreadsheet to calculate your profit, profit % and update your balance. Put any notes you want in the notes column.
When I use notes, I tend to note if an item buys instantly for less than I paid, or if it sells instantly for more etc. You can usually assume all items will buy within 36-48 hours, and sell within another 24-36 hours. If this doesn't happen, you can either cancel the buy, or update your buy/sell price.
Many might see this as a slow method of making money, but if you make a profit every sale you will start to see your cashpot rise. In just over a week I've increased my 500k to almost 700k.
Good luck and happy merchanting.
Note: I have only copied the formulas up to row 31. After this simply Copy/Paste the formulas onto more lines.Update: Google sheets version:
https://docs.google.com/spreadsheets/d/1saCXQyec3EIo1z64Tsj_ylLZ4-ttjs57v38LHRc8iH4/edit?usp=sharing