Confirm whether you have flexible or rigid flat foot. If arch disappears during standing, but appears when you keep your feet on bed or when you curl your toes or stand on tip toes, then you have a flexible flat foot. If arches do not appear in any of these positions, then you have rigid flat foot and you need to consult orthopaedician.
If you have flexible flat foot, the following exercises will help. (1) Place a newspaper on the floor. Sit on chair. Keep your heel on a fixed position on paper. Try to crumple the paper by curling your toes. (If this is difficult, you may use folded newspaper sheet to make it thicker. Or use towel / bedsheet instead of newspaper. After few days, progress to using single sheet of paper).
(2) Stand or walk on tip toes.
(3) If your calf muscles are tight, stretch them. Sit with legs kept on bed. Keep knees straight. Pull your foot towards you using towel or dupatta.
Health Tips
Use arch support inserts in shoes till the normal arch gets formed.