Well, the most important thing while making a portrait is to figure out the exact proportion of the object. The proportions matter a lot in the artwork, due to which many amateur artists who are good at sketching or coloring find it difficult to bring their art to life. Next is the tone, that is to use exact color as it is in the real world. ‘Hyperrealism’ is achieved when you master the concepts of proportion and tone of the object.
Coming to what we can do to improve our portraits and human figures are – first, observe the object carefully and try to measure and compare each and every feature like the distance between the eyes, length of the nose and also, the appropriate distance between your head, hip and feet. Here in the link below, the proportions of human body are nicely shown.
https://www.thedrawingsource.com/figure-drawing-proportions.html

For making realistic portraits, tonal gradient is an important factor. You must not go hard and rough in the initial phase, rather gradually go from lighter portions to darker shade. Here is a link which would help you understand the concept much better.
https://www.erikalancaster.com/art-blog/6-essential-tips-for-realistic-drawing

Hope these suggestions will help you make better portraits. 😃