Keep your phone off until you reach the end of this guide.
Locate the SIM tray on your phone (typically located on the side of the phone or under the battery). If needed, insert a paper clip or a SIM ejector tool into the SIM tray hole. Push gently until the tray ejects.
Note how your SIM is sitting in the tray, and take it out. Locate the SIM card included in the SIM Kit. (Pictured below)
Compare your current SIM to the inserts on your new SIM card. Pop out the correct size on the SIM card and insert the new SIM into the tray. If your phone was made after 2015, it will likely take a nano SIM, the smallest of the three sizes.
If you pop out the incorrect size of SIM, just replace it in the card and pop out the larger size.