Focus your thoughts on the person you want to no longer have feelings for.
Think of all the reasons of yours & traits of theirs that you can, to back up your decision.
Write this person's name on a small slip of paper.
Cross out the name with deliberation. Say:
You no longer hold a special place in my heart.
You are not the right person for me.
I will bury my feelings for you here & make a fresh start.
Bury this bit of paper & all your feelings about this person outside under a waning moon.