how can perform qaza namaz?

you can perform your kaza prayer all time except 3 kerahat times. in kerahat time,you can not perform any salat except that time's farz. after 45 minutes beginning from sunrise,45 minutes ago from noon midday ezan and sunset prayer call. while you perform kaza prayer,you must say ı designed to perform salat which is my first kaza...or my last kaza.because for kaza salat,specification is farz. besides 5 time salats,you must perform vitr's kaza,it is vacib. vacib is same with farz.only,a person who dennies farz be nonmuslim,but person who dennies vacib does not be nonmuslim,be sinful.

Qaza prayers are when you have missed a previous prayer in the given time limit so you can do it again, when you say your intention or niath at the beginning, instead of saying the normal you say that you pray Qaza of (no. of rakahs) rakahs of (name of prayer) and continue the prayer as normal. As to your second question I believe it means that if you have missed two prayers for example Fajr and Zuhr, you can read them together as Qaza since you have missed both.