Cut the decal out very carefully, keeping as near to the edge of the design as possible, and place in a saucer of warm water for 45 seconds. Slide the decal off its backing sheet and into position on the model. Using the tip of a paint brush to position the decal reduces the chance of damage. Once in position gently dab the decal with a piece of tissue paper to absorb any excess moisture.
When handling decals always take care as they are often small and delicate. To help soften decals and secure them in position you can use Humbrol Decalfix. Decalfix can be used by simply applying the solution with a brush or immersing the decals in the solution for 45 seconds and then sliding into the required position.
To stop your decals from cracking simply apply a coat of Humbrol Gloss Varnish to the area where the decals are to be placed. Once the varnish is dry you can then put the decals in the correct position. When the decals are in place and free from any moisture, coat in either gloss or matt varnish depending on the type of finish you require.