Be sure that you are calm yourself when you talk to your daughter. If you sound anxious, she will pick up on that. Tell your daughter in a matter-of-fact way that you believe that most people are GOOD, and that this means that most strangers are good, but that a few people have problems that might cause them to hurt kids.
Tell your daughter that she does NOT have to worry about strangers if she follows your safety rules. If your daughter is by herself, the safety rule is to come and check with you first before she gets close to or talks to anyone she doesn't know well. Help her come up with specific examples of people she knows well and people she doesn't.
If your daughter becomes separated from you, she needs to know what your safety plan is if she is for example, lost and cannot check first. In that case, she should go to the checkout counter or, if that doesn’t work, look for a woman with children to help her.
When your daughter gets old enough to be on her own without you, her safety rule is to THINK FIRST and to know how to GET HELP.
Remember that most of the people who harm children are not strangers but people they know. Be sure that you teach your daughter how to set boundaries with people she knows. Her safety plan is to check with you before she changes the plan about who she is with, where they are going and what they are doing.
Studies have shown that just raising awareness without practicing skills can raise anxiety. Practicing skills to rehearse how to handle different situations almost always reduces anxiety and builds competence. Have your daughter pretend that a friendly stranger is approaching her and have her practice coming to you to check first. Discuss and practice different safety plans for getting help from strangers if your daughter cannot find you to check first with.