There are several ways of doing this. You can make stuff up yourself, or follow an existing language. You can get books and teach yourself, or do a class.

We started at around 6 months. Generally they say babies are capable of signing once they have the coordination to clap. At first I borrowed a book from the library, and learnt a few basic signs - eat, drink, milk, mum, dad, that sort of thing - and started doing it.

It's easiest to introduce objects first. You need to hold up the object, do the sign and say the word all at once, in baby's line of sight. At first you may need to do this repeatedly before they get it, but once they do it gets easier.

It's really hard to get started because it can take a while for baby to do the signs. And they're fairly unco to begin with so if they start waving their arms at you at the right time you kinda have to guess that they're trying to sign something. It helps if others in the family also do the signs.

I started a class when DS was around 10 months, by which time DS had 5-6 signs. By 15 months he had around 70-odd. At this point he could only say mum, dad and cat I think.
The class helped me to focus my efforts. It was also really helpful that my mum and sister (and even one of my friends) got into it a bit, although DH was a bit slack about it at first.

It takes time to get started and can seem frustrating at first - but if you can stick with it, once they 'get' it, it becomes quite easy to introduce more and more signs. By 12 months or so they can generally pick them up first time you show them. The trick is finding signs they're interesting in communicating with you. Animals, favourite toys, food, whatever. You'll find that as they learn to speak they drop the signs in favour of the words too.

I know some people worry it impedes language development but studies show the opposite, so long as you always talk to your child as you sign. DS hasn't signed in quite some time now and never stops talking.