How does potassium and chloride combine?

KCl not sure if you meant that

They form ionic bonds

Potassium has 1 electron in its outer shell so it wants to lose it to become more stable.

Chlorine has 7 in its outer shell so it wants to gain 1 to be more stable

The potassium donates the 1 electron to Cl

This forms K + ion and Cl - ion