In a blender add in the chopped mango pieces blend it to a fine puree without adding water. Take saucepan/Kadai which you are going to use to prepare the jam. Transfer the mango puree into the saucepan add in sugar and lemon juice and allow it rest for about 30 mins. This will help the contents blend together.
Switch on the flame and start stirring the mixture in a low flame. It will be juicy at the beginning and slowly it will start leaving the sides of the saucepan, it starts to turn shiny and reaches the Jam consistency.
Once the Jam consistency is reached switch off the flame and transfer the contents to a sterilized bottle for longer shelf life. You can test the Consistency of the Jam if you take that in a spatula it should stick to it, similarly, you can test by taking that into a plate it should be firm it should not fall down. This is the correct consistency.
Notes
Always use a clean spoon for longer shelf life.
If you think your mangoes are not sweeter you can add more sugar.