When showing signs your baby might copy you so closely that he actually does the mirror image of what you do. Incidentally this will be incorrect, but also not a big deal so long as you understand what the sign means.
Take the sign for LION by example which is done by drawing the hand over the top of the head symbolizing the lion’s main. My son did the sign by drawing his hand forward, the exact way he’d see me do it. From his perspective you draw your hand away from him, so he does the same and draws his hand forward instead of backwards! If your baby does this, you know he’s really paying attention.
If you really concerned about this (and you probably shouldn’t) always be sure to model the correct sign. Next you might sit to his side and do the sign again several times. Next, correct your baby by moving his hands for him (called physical prompting). If your baby shows resistance to this, then don’t bother. In the end, it’s really not all the important. If you plan to continue with signing into your child’s early years then eventually he’ll figure out the right way of doing things on his own or you’ll be able to explain how the sign is really done verbally.