Stockpiling refers to the process of saving up your money on hand rather than spending it on military in order to make a big NW jump near the end of a game for a final boost in NW. Stockpiling can also be in the form of food or tech as well.
Stockpiling is needed near the end of a game because it allows a country to avoid paying for excess expenses if it were to spend its money gradually. Stockpiling is also need becaose of an existing bug, also known as the 2B bug, which doesn't allow you to have more than 2B cash on hand at any time. Any gain in money will be lost beyond the 2B in your hand. Therefore stockpiling in the form of food or tech will allow you to stockpile more than just 2B cash in hand.
For tourney/standard games, if you are just an averaged player, the best time is to stockpile after 3/4 of the game. Hopefully by then you have reached a good acreage already (see General Tips). For more advanced players I would recommend to start after 2/3 of the game has passed, depending on whether you have reached your goal of acreage already.
Getting your first 2B cash:
This is nothing hard. For whatever your strategy you are doing (indy, casher, techer, farmer), save up the cash you got from sales or direct cash production until you have 2B cash.
Stockpiling beyond your 2B cash:
This is something that takes some practice. The idea is to stockpile in food and tech and sell it near the end to get your cash back. The key is to get your stockpile goods as early as you can (if you know that eventually you'll get more than 2B cash), preferably at the very start of your stockpile process. You want to buy your stockpile goods low so that when u sell your goods, you will hopefully sell at an equal price (at minimum) during the market crash to get back the cash that you originally invested in. A common technique in stockpiling in food is to put them on the public market at some outrageous prices such as $300. The reason for selling the food and not keeping is that food diminishes at a rate of 0.001% of total bushels per turn. If you have a large amount of bushels on hand at once, this rate can really hurt how much you get stockpiled. Secondary reason is that some people like to guerilla attack countries which destroys a large amount of food (if you have a lot of food on hand). With about one week or less remaining in the game, you want to stop stockpiling and considering spending some or all of your money, as well as, selling your food on either private or public market in order to generate further revenues. Do not exceed 2 billion dollars though.
Techer
For techers, since the tech market crashes gradually, you would want to liquidate your tech early and stockpile in the form of money or food since your tech will be selling at lower and lower prices as the game progresses to the end. Some people would try to continue to tech without liquidating their tech early. That's a big mistake.
Farmer
For farmers, sell your food as frequently as possible to reduce food decay and to reduce the chance of losing food by attacks from others. Keep selling (preferably on private market to ensure that the food sells) until you have 2B cash. Sit with the 2B cash in hand and continue to stockpile food day after day. Put all your excess food on the market at the end of each day (at some high prices like $200 to make sure you don't have food on hand to lose and at the same time the food won't sell). When the food comes back after 72 hrs, if it's still not near the end of the reset, put them back on the public market. Keep doing this until the 5 or 3 last days, and then sell all your food on your market for money to generate the maximum profit/return. (if u can no longer sell on market then go on private and sell it).
Casher
If you are a casher, then you simply cash your turns each day. Spend turns cashing until you hit 2 billion dollars then, buy food and then sell it on the public market at an outrageous price like $300 in order for it to not be sold.
Industrialist
If you are an industrialist, then stockpiling can be quite challenging. Unlike casher and farmer, you are not generating anything easily stockpilable. Each turn you are producing military, and most likely are loosing quite a large sum of money each day. The method of stockpiling as an Indy is as follows: Start off by selling your jets on the public market in order to lessen your expenses (since jets are not needed anymore). Then begin selling the maximum amount of military you can each day of whatever unit you are producing. It is best to produce 100% turrets while stockpiling. Produce spies/tanks/troops for sake of needing those units for protection if you feel you need them. But once you have enough produce 100% turrets for the rest of the round. The reason of this is they are the best for the NW when sold on private market and are low on maintenance. Once you reach 2 billion dollars, begin to stockpile bushels. Sell the bushels on the public market at an outrageous price like $300 in order for it to not be sold.