How can I prevent giving my partner cystitis?

Unlikely to transmit. There is no reason why you would give your partner cystitis. It is not transmitted from one person to an other even if hey are intimate.
Hygiene. Excellent hygiene for both partners and hand washing before relations is important. Also, all sexual toys will need to be clean and dry before use. Adequate lubrication will lead to less irritation and less infection. If still a problem, cranberry tabs may stop infection after relations.