T-shirt Estimation in Agile is one of the simple and easy ways to start with the relative estimation. it often found that T-shirt estimation is not much recommended because of its major disadvantage of individual perception of sizes. Like My Large size may vary with yours. Practical example will be American large (L) is Extra Large (XL) for Indians.
In most of company they follow certain rules to avoid these conflicts. They define numbers or range for the Sizes.
The way of estimation carryout is same as of planning poker see here. But here we used T-shirt sizes instead of numbers for better and easier to estimate.
T-Shirt Estimation enables communication with Product Owners. PO can easily understand that a Large size of work is going be to bigger than a Medium size of work yet smaller than an XLarge size of work.
For new teams it’s OK to start but as team gets familiar with the estimation then better to switch to other estimation techniques.