How to keep the house cool from inside by covering the window panes?
I see you're writing from India, so the standard US answer of installing light blocking cellular blinds may not apply to you.

Consider using a thin material that reflects light instead of absorbing much of it, such as aluminum foil.  If you want it to look nice, you can stack the foil between a light piece of paper for the outside (so the neighbors aren't blinded by the reflection) and cardboard (for rigidity).