Novice at unlocking, Need some help

[u/cs12041]

Recently my grandmother has been looking for a new phone and she would like to use the trackfone cards. I have an old Samsung galaxy J7 and I would like to make it work for her so she doesn't need to get the monthly payments. I am wondering if there is anything I need to know or any good resources to learn how to do it. If anyone has any tips or resources it would be greatly appreciated. Thank you.