Get rid of any existing insulation
First of all you need to get rid of old insulation from the attic. You can do this with the help from builders vacuum which can be rented from any hardware store. You need to get an air purifier till the finishing of attic walls and ceilings.
After removing old insulation from your attic, you need to get new insulation rolls. Install these insulation rolls between the wall studs. You can use fibreglass batts for insulation purposes. Cut the rolls after measuring the length between wall studs and then place the insulation.
Install electrical wirings
Next you need to place the electrical wires on the ceiling. Remember to place it on the right places and run it towards the electrical outlets.
Install sub-floor panels
After fixing the electrical wiring on the ceilings, you need to place the sub-floor panels on the attic floor. Remember to cut these panels according to the appropriate sizes and then place over the beam.
Then you need to hang drywall with the help from drywall screws. Later use the drywall tape over the closures. In the end, use the drywall mud and place over the tape.
After hanging drywall, paint the walls. Apply around three coats and choose the colours which you desire.
You need to install flooring after painting the walls.
In the end, simply fix the switchboards, light fixtures and use caulks to cover open spaces.