You can't.
And some prepaid cards are not accepted by Paypal at all.
Either use a 'real' credit card to pay or attach your bank account to the PP account and pay from there.
You should be aware that the bank account solution, while perfectly safe, has two drawbacks. You are in effect writing an e-cheque to your seller and while the money is immediately removed from your account, it may take up to 10 days to clear the Canadian and US* banking systems. And should things go pear-shaped, which is unlikely, your bank account ** does not offer the secondary layer of Buyer Protection that the 'real' credit card does.
*Because PP is an American company. Even if your seller is in China, an echeque passes through our own and PP's national banking system.
** Neither does the prepaid card for that matter nor a PP balance which some sellers accumulate and pay from. The 'real'credit card is fastest and safest.
