Kiss the feet or the hem, as appropriate to the icon. Failing such, the bottom of the icon is a safe choice, especially for group scene icons, like the Icon of the presentation of the Commandments.
With saints, you may kiss the hands, but most of the people I know do not. With icons of Christ, I’ve been taught that one kisses the hands only if there is no hem nor foot.
Icons of the Crucifiction, most of us kiss the base of the cross.
Typically, make the sign of the cross twice, kiss the icon, kiss it again.
However, certain venerations are much simplified… for example, many kiss their fingers and then place them at the bottom edge of the Icon on the tetrapod as they pass while in the communion line.
Icons on the Iconostas are not kissed. (they are harder to clean and expected to last quite a while…)
The priest’s blessing cross (itself an icon), usually the feet of Christ are kissed.