A driving anvil
A ground sleeve with a driving anvil inserted in the end
Anvil slides in ground sleeve, ground sleeve gets placed in the ground. Ground can be loosened up with a breaker bar if very rocky or stiff, but often not necessary. Tap the anvil with a sledgehammer (try and drive straight, can use a small magnetic level to make it easier) until the ground sleeve is about 1" from grade.
A post inserted into a ground sleeve
then slide the post in the sleeve, which is now in the ground. The post/sleeve fit is very tight but to prevent it from spinning, you can use some very small nails- hammer them in between the sleeve and post, creating a wedge.