Before applying glue to your wall, you want to mark the hanging spots, making sure your poster is level.
Next, load up a glue stick, plug it in and let it heat up, then squeeze a bead of hot glue onto the wall where the top corners of your poster will be.
While the glue is still warm and soft, grab the poster and align it with your markings, pressing the corners gently onto the warm, sticky glue beads.
Your poster should hang securely from the two corners within half a minute or so.
Next, squeeze out two more beads on the wall, under the bottom corners of your poster. Making sure the edges of the sheet are straight, press each bottom corner onto the glue, making sure the poster is firmly attached to the wall in all four corners.
Since you're working with a fairly hot tool, please make sure kids aren't around when you do this.
Also, it's a good idea to have a second person helping you out, since some posters can be quite large!