If you've never installed a vinyl decal it can be a challenge.  Getting it on without tearing it or losing pieces just takes a little patience.  This is the process I use.

When you get the decal, it will have a paper backing, the vinyl decal on top of that and then a clear plastic layer called transfer tape.

We try to do this first optional step before it leaves but you may want to do it again.

Optional first step: put decal face down on a table, press really good on the back with a squeegee or thick card.  You can try to very slowly peel the backing paper off from each corner to check if decal will come off paper.  If any part of the decal stays on the paper, put the paper back down and press real good on that spot.  Once you’ve checked decal for detaching, put paper back down.  This will act as a pre-release of decal and make transfer easier.

1.  Clean the window where decal will be installed.  Choose an area that does not have a window wiper and doesn't obstruct your view.  Preferably not a roll down window as the rubbing against the seal may cause wear.

2. Put tape (any type easy to remove) on one edge, depending on which edge you want to work from.

3. Position decal on window using tape as an anchor for placement.

4. Once in position, lift up decal and peel off either the whole backing (small decals) or a portion of it.

5. Using a squeegee (or equivalent) push decal from taped edge onto surface.

6. Use the squeegee to go over it multiple times to make sure the decal has attached to surface.

7. Pick a corner to start peeling off the transfer tape.

8. Peel the transfer tape off slowly along the surface.

As you get to small details, make sure they are stuck to surface and not the transfer tape.

Detached areas or small details will have a greater tendency to stay on transfer tape and may tear or get lost.

9. Don't wash the window for a few days to allow the decal adhesive to set.

Should last you up to at least 6 years in normal conditions.