just like a puppy, I am assuming you have a going out routine? you know, be a good girl, I won't be long, give small treat and so on?
If not, then start one so she knows what will happen. Something like a stuffed kong or a safe chew that will keep her entertained when you go out is a good idea too.
So you start off on a day and time when she has had exercise, has emptied and is relaxed and ready to have a nap. Do your going out routine, go out of the front door shut it then turn round and come back in again. Greet your girl calmly and tell her what a good clever girl she is...and that's it for the day. Once she is happy and calm with this then start to leave her a little longer, literally 30 seconds or so building up to a minute, then walk a little way away and so on. keep building slowly always after exercise and when she is ready for a nap and she will get the idea that its no big deal. If she poos or starts to look stressed, give it a day's rest, go back a few stages and start again. Once she has the idea nailed that being left alone is not an issue, you can start to leave her for longer. Always make sure that she has had exercise and a chance to empty first.