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The best method is letting juniors or interns to practice and experience the task/job, even if there is a room for error.
The best method to do something right,
is to do something wrong... and learn from it.
I agree with the majority of responses and wanted to add an outline we used in our personnel development training on the5 tips for training new employees:
1. Have a Variety of Different Training Methods; 2. Take Advantage of Training Opportunities; 3. Let Current Employees Help; 4. Be Aware of Skill Level Differences; 5. Have Some Fun!