Build up the relationship gradually. Don't make them fall in love too quickly. Maybe start with them as reluctant allies, then friends, and finally lovers. Include some conflict, like Koga's loyalty to his pack conflicting with his feelings for Kagome, or Kagome's connection to Inuyasha getting in the way at first.
First, you need to understand the characters well. Koga is a proud wolf demon and Kagome is a kind - hearted and strong - willed girl. Keep their personalities consistent. For example, Koga would be bold in his advances towards Kagome but also respectful of her strength.