Rather than telling why you should look more confident? Let's have a conversation between you and Me. I will refer myself as 'Motivation With Kailas (MWK)':
You: How can I look more confident in a few minutes?
MWK: The first minute, regardless you are sitting or standing, do make your poster straight now. Initially, you may feel little stiffness, but you can take 2-3 deep breaths to be relaxed and straight.
You: Thanks, I am comfortable. How do I know that my posture is straight?
MWK: Nice question. First, make sure that your weight is balanced on both legs and toes are making 45 to 60 degrees angle. Then, while looking straight, you are not able to see ground.
MWK: Getting straight posture, you can surely do it in one minute. Let's move on to the second minute, which is “start walking a little faster than you normally walk”. Make sure that you do not look in a rush but just a little faster.
You: Why should I walk faster?
MWK: Walking straight will make your posture straight. You will look more energetic and enthusiastic. At the end of the day, you have saved a lot of time which you can use to read at least two pages.
You: Yes, I'm excited, please tell me next point.
MWK: Glad to know that you are excited. Next point is about handshake. Whenever you are making handshake take control of it.
You: What do you mean by taking control?
MWK: Press other person's hand slightly, he or she should get a feeling of a warm touch. Then, follow the literal meaning of a handshake, shake their hand at least twice and not more than thrice.
You: I'm afraid, it may look awkward.
MWK: I can understand, why don't you try this with your friend now. Then, ask for feedback about this handshake.
You: Wow, sure. I can do this.
MWK: One more bonus point about handshake is while leaving from the conversation you can use your other hand to cover handshake. This gives warmth to your relationship and shows bonding.
You: Yes, looks like this is optional and situational.
MWK: Awesome, you got my point.
You: What's next?
MWK: Smile and initiate the Conversation. Make sure that this Conversation starts with the word "You" like 'You are looking awesome. How are you?', 'You are rocking. How's your day?'.
You: Why should I start Converse?
MWK: Initiating the conversation gives you inner confidence. This allows you to take control of the conversation about its context, positivity and time.
You: Yes, I do agree about initiating the converse please tell me more about it.
MWK: I know, you must be using magical words - 'Thank you' in your conversation many times. Now onwards, I want you to make a little change in this, rather than saying only "Thank you", say "Thank you for".
You: Why is it?
MWK: This is to add a reason why are you thankful. This will give clarity to that person and yourself too. For example, 'thank you for having a conversation', 'thank you for writing that report'. Or simply, 'thank you for everything', 'Thank you for all this'.
You: I am happy to know these steps to be more confident. How can I recall those steps?
MWK: Simply, act on this script. Think that, your imaginary partner is here. You get your posture straight, walk a little faster toward her, have a handshake with a smile and say "You are very prompt about time. Thank you for coming".
You: Sounds great. How can I make it as my habit?
MWK: Yes, that's our goal to make you more confident in your daily life. By experience, I can say that, initially, it will look little robotic to follow above points, but I am sure it will really work.
Please try this activity and let me know your feedback, comments.