Very carefully

Side windows:
A very small bead of adhesive is applied to the edges of the plastic serving as the window. The window is placed and taped in place. The tape stays in the plane . (its inside the fuselage) You have to be careful that there is no excessive glue... or it will get pressed out thecrack between window and frame.

Front windscreen:
I tend to use a similar method as with theside windpws... but I use trim tape on the outside. (the correct width for the bolt-on frame that retains the glass in the full scale.... if applicable)

*******

If going for contest quality scale detail... try to look at a full scale aircraft of the same type and see how they affixed the windshield. simulate that with your installation. (If the windshield is slotted under the front deck... you need a slot to insert yours into or you won't be able to get it on looking correct.)