By "Instacart", I assume that you have a side job delivering groceries and the like.
Because you also have a W-2 job, you need to estimate what your net 2021 earnings from Instacart will be. This is because, it is not uncommon for taxpayers to adjust their withholding on their W-2 to cover any taxes from the side gig, especially if the net income from your side gig is modest. In this case, if your W-2 withholding is enough, then you won't need to file quarterly estimated tax payments.
On the other hand, you have the consideration of how side gigs are taxed. You owe federal income tax on your net Schedule C income, but you also owe self-employment taxes on the same net Schedule C income. This can add up rapidly if you actually net a profit - at a minimum, 25.3% (10% federal income tax and 15.3% self-employment taxes).
Of course, we in the Community cannot see your private tax data, but why don't you go to the IRS' Tax Withholding Estimator and see what it estimates that your tax for the year might be, and then adjust your withholding with your employer so that it matches.
If this doesn't seem to give you a good result, you can do what I did as a consultant - set aside a certain percentage of your net Schedule C income (yes, this means that you will have to review income and expenses each month), and then pay that amount quarterly.
As for how to make estimated tax payments, in the past you would have mailed in a 1040-ES with a check each quarter, but now you can pay online and mark it as for estimated taxes. Please see this IRS webpage.
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