The best time to ask for a referral is the ten minutes after you pack up.
The job's done, the client is standing in a finished room admiring the quality of your work and pleased with her interactions with you. She'll never be happier with you than she is right now. By the time the invoice lands, she's moved on. Seize this moment.
Three things to do before you drive off.
Ask one specific question:
"Is anyone in the street talking about doing their bathroom?" this gets a name. "Tell your friends about us" gets a nod or a non-committal response.
Leave something with your details that she'll keep. Use a gift that suits the age demographic of your client. It doesn't have to be expensive, but it should be beneficial, so she'll keep it an easily accessible and visible place. A fridge magnet stays on the fridge. A business card goes in a drawer.
Ask if you can take a photo of the finished job with the crew in it, then send it to her that night. Make the image relaxed and ask if your client would like to be included. More often than you'd expect, it ends up in a group chat or on her socials.
That last one only works if the shirt you are working in in the photo says who you are.
Worth checking what yours says on a phone at arm's length. Make sure your business information is easily readable. If not, it may be time for a redesign.