It is very important to wallpaper at 18-20 degrees and to allow drying at room temperature (to reduce the possibility of shrinkage).
A smooth surface is also a must, and if it has already been painted, it is better to sand it lightly first, so that the glue is absorbed better.
When hanging multiple strips: hang them edge to edge. Visually match the pattern on the wall then firmly press together so that the seams are not visible.
Tip: Paint the wall a light uniform colour if it differs significantly from your wallpaper as dark patterns / colours may show through. For darker wallpaper you can opt to apply each 50 cm a darker colour strip, just in case a small seam might become visible.