Agile Scrum Software Development Process

I will share my experience here. There are different ways how we can estimate user stories. Below two ways are I have experienced.

1. T-shirt Sizing (Small, Medium, Large, Extra Large)
2. Story Points (1, 2, 3, 5, 8....)

And if we want to map T-shirt sizing and points it will look as below.

Small - (1,2,3 points)
Medium - (5, 8 points)
Large - (13, 20 points)

Now the tricky part how we should assign. Here experience comes into picture and they know the application and effort might required for the user story. It is always good to make small stories.

Now when you have combination of fresh and experience people, we might need to depend on the experience one and freshers will learn from them and as they get more use to system and the implementation part you will see them estimating it very well.

No one can be 100% sure about estimation because of unknown issues which can come. So best practice is have patience and give freshers a chance to understand and estimate with help of seniors.