Most income in the United States is subject to withholding. What that means is that your employer automatically deducts taxes from your paycheck based on the tax status you claim. There are a lot of different types of income though that are not subject to automatic withholdings like prize money, lottery winning, interest or dividends on investments, alimony, rent, gains, and self employment. Estimated tax is that amount of tax paid on both income that is subject to withholding and income that is not. If you do not pay enough estimated tax, you are subjected to penalties.
In order to use an IRS Estimated Tax form there are things you must first consider. Are you even eligible to pay an estimated tax? If you expect to own more than one thousand dollars after your withholdings, you are eligible to pay estimated tax. This holds true for individual, sole proprietors, corporations (although for corporations the amount is $500 rather than $1000). If you have a refund due to you, but expect that you will owe the following year, you can have your refund applied to your next income tax return so that you do not have to worry about it. This is another form of paying estimated tax.
Not everyone will need an IRS estimated tax form. If you are a U.S. citizen, your taxes are for the previous 12 months, and you have no prior tax liability (you do not owe previous taxes) then you do not need to pay estimated taxes. There are different requirements based on whether or not you are filing as an individual or a corporation so be sure to check with the person who does your taxes for you.
In the event that you are filing your taxes first and want to use an IRS estimated tax form to figure what you need to pay the first thing you will need to do is figure out your income (this includes tax already paid, credits, and actual taxable income). This is where the form comes in handy. The form allows you to keep track of these numbers in an ordered manner that makes sense. You need to check when your estimated tax is due also because there are four times yearly to pay this and you need to know what date to pay. Like most tax situations, if you are planning on using an IRS estimated tax form to help with your taxes your best bet is to pay someone to do it professionally.