Great!
1. If not already created, create the custom column in the list that will fire the work flow. I recommend using a Choice column instead of Yes/No because it's easier to work with in Designer.
2. Open Sharepoint Designer and connect to the site where you'd like to create the alert to take place.
3. Select File, New, Workflow.
4. Give the work flow a name and specify the appropriate list or library this is to be attached to.
5. Check to enable the boxes that read Automatically start this workflow... and click Next.
6. Add a compare field option to the condition. Select your column by clicking on the field hyperlink. Set the value to Yes by clicking that hyperlink.
7. Add a Send Email option to the Action section. Build the email so that it contains any information you may want. To add the AssignedTo user, click the address book by the To line. Double click Workflow lookup and select Current Item and your AssignedTo field.
8. Check the workflow for errors and click Finish. The workflow will build and then be available on the list.
When a new item is created or updated and the Notify box is marked as yes, it will email the Assigned To person.