This may already be common knowledge, I thought I'd post the method I use to make decals for my models. I'd also be interested in how you make yours. I am aware of the inkjet printable decal paper, but as it needs overcoating, i've tended to avoid it.
Step 1.
Find or design your graphic. Size and print it with your fav graphics package. Print it and laminate it with a normal document laminator (doculam).
Step 2.
Cut it out and trim it to size.
Step 3.
Now carefully with a sharp blade (mind those fingers) peel off the back side of the doculam plastic. You will find it comes away easily and leaves the print stuck to the other side of the doculam. In my pic the waste side is the one I slit with a knife just to show.
Step 4.
Now attach thin double sided tape to the now really thin decal.
Step 5.
Trim off the excess again
Step 6.
Remove the tape backing paper and apply to your model. To be truely waterproof you would need to run a thin brush with clear cote around the very edge to stop water ingressing.
Doon't try gluing instead of tape. It just makes a mess of the inked paper (I tried)
Step 1.

Find or design your graphic. Size and print it with your fav graphics package. Print it and laminate it with a normal document laminator (doculam).
Step 2.

Cut it out and trim it to size.
Step 3.

Now carefully with a sharp blade (mind those fingers) peel off the back side of the doculam plastic. You will find it comes away easily and leaves the print stuck to the other side of the doculam. In my pic the waste side is the one I slit with a knife just to show.
Step 4.

Now attach thin double sided tape to the now really thin decal.
Step 5.

Trim off the excess again
Step 6.

Remove the tape backing paper and apply to your model. To be truely waterproof you would need to run a thin brush with clear cote around the very edge to stop water ingressing.
Doon't try gluing instead of tape. It just makes a mess of the inked paper (I tried)