Hello!
You can find the link to donate in FAQ 3 on this page: FAQ - YGO Omega
Once you have donated, send a PM to any of the members of the Duelist Unite Discord who have the Guardian Role or the Omega Role with your receipt in order to be validated and gain your rarities.
Thank you in advance for helping to maintain the YGO Omega servers and help make Omega be the best it can be.