There is a sex guide and puzzle guide here, but here's a summary of what it says about Smeargle specifically:
- Rub seven of Smeargle’s sensitive spots (arms, chest, belly, neck, head, ears, hat, calves, thighs, feet)
- Monitor his arms to know which areas are okay to rub
- Rub one of Smeargle’s sensitive spots for about 5 seconds continuously
- If his horniness meter is in the green, rapidly rub his hat
- If his horniness meter is in the yellow or red, rub his cock