You "model" sharing. With ours, this is what we do:
Me (Handing them a toy): Here, I'm sharing (xxx) with you.
<Then we get them to say (or sign) THANK YOU.>
Me (A little bit later): Can you share (xxx) with Mommy, PLEASE? (Pause. Give them a chance to respond either yes or no).
If they hand back the toy, say/sign THANK YOU and heap praises. If they don't want to, don't forcibly take it away. We just say, "Okay, you want to play with it a bit longer, huh? Maybe you'll share with Mommy later." Then you try to ask again. You can also demonstrate sharing again by handing her another toy and say that you are SHARING that toy with her.
We also do a show of sharing things between me and my husband where we politely (emphasizing use of PLEASE) ask each other for things hand each other objects and say/sign THANK YOU when we receive the object.
Please note that it is normal that there might be occasions when they simply would not want to share. Respect those times, but on occasions when they do share, praise them to reinforce the positive behavior.
Good luck!