The first step in being a self-assured person is thus: Highlight your positive points. It could be you are attractive, athletic, intelligent, academically gifted, kind, family-oriented, independent, etc.
Highlight your positive points is key to being confident and self-assured, since confidence is essentially pride. It is pride in who you are, and pride in the FACT that you are a worthwhile person.
Once you have identified your positive feature, stress on it and emphasise it. If one is, say, academically gifted, then conduct some research for a university, or teach a class of students. This would enable you to feel even more confident, since the further success you would receive would enhance your feelings of self-worth even further.
Being self-assured is also part mental.
This involves ACTIVELY thinking "I AM worthwhile! I AM somebody! I AM capable!"
Thoughts affect actions and emotions, and vice versa. Consistent patterns of thoughts also become ingrained and second nature over time. A re-orientation of thought processes is thus a key component of a PERSISTENT self-assured attitude.
Do not let slights affect you.
This may seem difficult at first, but over time it can become easy. A good way of accomplishing this is thus. If somebody says something negative to you, analyse the statement and ask yourself if it is true. If it IS true, then ask do you care. If you DO care, then ask yourself why should it be changed. And changed to suit whom? No human being has total control over another. Why then should anybody change to suit another's whim?
Moreover, nobody has to acknowledge anybody else's opinion, even if a person holds a right to that opinion. Mentally deflecting negative comments in this way can greatly enhance your new found self-assured attitude.