How to expose LWC to flow builder

To make the components available in flow screens, edit the lightning web component configuration file to include a target of lightning_FlowScreen. For example:

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
  <apiVersion>45.0</apiVersion>
  <isExposed>true</isExposed>
  <masterLabel>Example Flow Component</masterLabel>
  <description>This lwc is available in flows.</description>
  <targets>
      <target>lightning__FlowScreen</target>
  </targets>
</LightningComponentBundle>

See the Winter ‘20 release notes for more information.

As per the documentation:
https://developer.salesforce.com/docs/component-library/documentation/lwc/lwc.get_started_supported_experiences
LWC is not supported in Visual flows as of now!

As mentioned, you can wrap a lightning web component in an aura component, and can use the same in flows, and in many other experiences and tools where LWC is not supported!

Update: Now LWC is supported in flows from Winter' 20 updates, as the feature has been implemented.
https://releasenotes.docs.salesforce.com/en-us/winter20/release-notes/rn_forcecom_flow_fbuilder_solutions.htm

Winter '20 Salesforce release will allow you to embed Lightning Web components to flow builder.