What is meant by 'demineralised' water and how can it be obtained?

(N/A) Demineralised water is water that is completely free from all soluble mineral salts (cations like $Na^+$,$Ca^{2+}$,$Mg^{2+}$ and anions like $Cl^-$,$SO_4^{2-}$,$HCO_3^-$).
It is obtained by passing water successively through a cation exchange resin (in $H^+$ form) and an anion exchange resin (in $OH^-$ form).
$1$. In the cation exchange process,$H^+$ ions exchange for $Na^+$,$Ca^{2+}$,$Mg^{2+}$ and other cations present in water:
$2RH_{(s)} + M^{2+}_{(aq)} \longrightarrow MR_{2(s)} + 2H^+_{(aq)}$
$2$. In the anion exchange process,$OH^-$ ions exchange for anions like $Cl^-$,$HCO_3^-$,$SO_4^{2-}$:
$R'NH_{2(s)} + X^-_{(aq)} \longrightarrow R'NH_3^+X^-_{(s)} + OH^-_{(aq)}$
Finally,$H^+$ and $OH^-$ ions react to form pure water:
$H^+_{(aq)} + OH^-_{(aq)} \longrightarrow H_2O_{(l)}$