The two materials you will need are bristles strips, and compressive foam seals.
First open the window. Put compressive foam seals on the underside of the top and bottom edge of the window (Marked A). When doing this it is important that you cut the strip to the exact size required to provide as complete a seal as possible. For wider frames it may be possible to install a second layer of compressive frame seal.
Next, close the window, and fit bristle strip to both sides of the beading of the lower sash (Marked B). Do exactly the same for the upper sash BUT on the outside.
Finally to draft proof where the sashs meet (Marked C). Bristle strips should be fitter on both sides with the bristles facing each other just touching. This will stop drafts coming in the middle.