Which direction should I slide my hand?
You should slide your hand outward, away from the center of your body. If you are right-handed, slide your hand slightly to the right. If you are left-handed, slide it slightly to the left.
Can I just sign the letter 'E' twice in the same spot?
While you can, it is not recommended. Signing the letter twice in the exact same spot can be difficult for the receiver to catch. Sliding the letter makes it clear that it is a double letter and improves your fingerspelling flow.
Does this sliding rule apply to all double letters?
The sliding motion is common for many double letters, like 'ee', 'oo', and 'll'. However, some double letters are indicated with a slight bounce or a double tap, depending on the handshape. For 'E', the slide is the standard method.