Welcome home to our wonderful faith!
I agree with SteveVH’s advice. I would imagine that the LDS would inform your parents once you formally resign, so if you want to be the one to tell them, you might want to do so before they hear it from their LDS leadership.
We converts usually advise our fellow converts to tell loved ones something along these lines.
First that you are grateful to them for bringing you up in a household dedicated to faith in God.
Secondly, that you deeply appreciate all they did for you and that you still love God.
Thirdly, that you want to keep your relationship with them and hope they wish to do the same.
And finally, that you will not be imposing your new beliefs on them and hope they will respect your decision to become Catholic. (Of course, this does not preclude you sharing your faith or answering their questions.)
This usually smoothes things because it will be a shock for them and may create fears for your salvation in their hearts, so you want to allay their fears and let them know you’re still their beloved child.
God bless you. Please let us know, if you wish, how things go.