Before anything, I want to tell you its going to be a long game, so be ready for it. May be test of your relationship as well. So stay strong and hold each other together.
First of all understand their view about it very clearly. Are they open to get you married to a partner of your choice or not?
Many time, parents gives us a feeling that we are open but actually they are not. This puts us in bad state.
Based on their state, try to navigate accordingly. Don’t be adamant or agressive while dealing with them. Handle them politely, assess their concerns carefully and try to clear them one by one.
Evaluate views of both parents, mostly one will be driving/influencing other, so try to make them understand.
Put the bright side of your partner in their front and make them resonate with them.
If they say something about your partner, don’t be defensive or aggressive, but politely make them understand.
Keep trying your emotional card, go out with them, give them time, care more for them, show they you are ready and matured enough to take a decision.
Involve your friends as well, they play good role when talking about your partner.
But be sure that that you never have huge fight with them.
Before doing all of this, be sure that you want to be with your partner, at no point you both can losen the grip.
And I will suggest that keep courage to move away from your partner as well.
Introduce your siblings/cousin to your partner and ensure they have good bond among them. This also helps in simplifying things.
All things from my personal experience.
Taking harsh steps will not let you be happy in any state, so keep yourself sane and strong.
Wish you all the best!