use light weight fusible interfacing (heat N bond lite is a good one- so is steam a seam lite & wonder under) as for the *windowing* that is your choice- you can draw your applique shapes onto the paper side of the fusible, then press to the wrong side of the fabric- cut out on the lines, fuse into place then stitch down...I use a machine blanket stitch, some people use a small zigzag- and some satin stitch...fold a piece of scrap fabric & try out different stitches & adjust size until you are happy with the stitch. it is more work to *window the fusible since you have to draw the shape, rough cut on the outside- cut on the inside of the shape a 1/4"-1/2" , then fuse into place & cut out the shape- since the fusible washes away when the quilt is laundered I never worry about it & spend the time on the extra steps- but it is a personal choice- again- do a little practice piece with both ways- & do it the way you like.