Casement windows are the ones that swing out or crank out, you either puddy them in or you use some kind of metal or plastic snap-in bead, if there puddy jobs then you need to arm yourself, you put on some gloves, ware glasses or goggles for your eyes, ware a long sleeve shirt and paints, use a small flat tipped screw driver not too sharp, and use a small size hammer, carefully beat the puddy out of the metal without destroying it, you kinda just hit on the edges of the old puddy with the flat tipped screw driver and it will all come out gradually.
If you decide to measuring these kinds of windows without busting out all of the puddy first you can do this : take the screw driver and hammer and just beat out a little pocket of old puddy on booth sides of the width and height and you can usually see the edge of the glass so you can measure it, and after you get it all beat out you want to confirm that, that size will work and not be too loose or too tight.
if there is snap-in bead, you carefully remove without destroying them, take a thin puddy knife and put it between the glass and the bead and lightly push it in, lift it up and gently pull it out without breaking it.
when measuring you want to leave about an 1/8 th of an inch gap all the way around, with snap-in bead type you have to have this 1/8 th of an inch gap to hold the snap-in bead in, especially on the top and bottom, too much gap on the top, the snap-in bead wont stay in, no gap on the bottom the snap-in bead wont go in, it has to be exactto take the broken glass out of a vinyl dual glazed window, after you have taken the snap-in bead off the front you have to go to the inside of the house and use the same puddy and break the seal around the glass and the frame.
to do this you put on some gloves and just push the puddy knife in-between the glass and the plastic window frame, and push the puddy knife all the way around the glass unit and do the bottom last that way you can catch it so it doesn't drop on the ground and you have a big mess that you have to clean up, you want to save as much of the broken unit as you can for information to have a new one made .
here is an old wood sash window that need a glass and some chafing scratch off the old paint and put