How can I fit the double glaze glass into the window frame?

take the beading out,but really if you are asking this how do you know what size unit to install ?

Its not expensive to get a window fitter out to do it, the glass is easy to break if you havent got the packers set right. If you are doing it yourself, have you got the right sized unit, remove glazing bars, with something like a putty knife, remove old unit, install new, including spacers, even the glass up so you have equal margin,if its an opener, toe and heel. Refit beads, these will be tight and need hitting in with a plastic/rubber hammer. You will know if the packers are not sat in right, when you hit the last bead in, crack.