If you are a member of the KDE community who has contributed to promoting, developing or documenting KDE or doing any other KDE related work, you can join the KDE e.V. as an active member. People who join the association do so according to the statutes of KDE e.V.. See the instructions for more details on how to join.
If you are a corporation and want to support KDE you can become a supporting member.
One-time For a one-time donation, the donations page can be used for any amount through a wide range of payment options.
Recurring If you want to support KDE by regularly donating money, consider becoming a Supporting Member. The donations page has additional information on your options.
KDE e.V. does not accept donations of "crypto" coins: Bitcoin, Etherium, Litecoin, Dogecoin and all the others. The tax regime that covers German not-for-profit associations considers those coins a form of speculation which would endanger our status -- so regretfully you cannot donate them directly. Cash out and then use a traditional method to donate.