Awkward Decals
The best answer is indeed to use decal softener. There are at least 3 different brands: Micro Scale (who invented it), Humbrol and Tamiya. Humbrol is most widely available, but Micro Scale is best.
All work the same way: a softener solution and a setting solution. All work best on a gloss base, and none harm the paint. You paint the softener on to the model where you want the decal to be, while the decal is soaking. Then place the decal and leave for a few minutes. Then press it down with tissue, tease out any air bubbles or wrinkles, etc. Apply more softener if necessary. After that, apply the setting solution over the top. Once completely dry, varnish over the top to protect and preserve.
Don't float the decal on water as suggested above - you're just washing off the adhesive and doing nothing to soften the decal film. The softener solution allows you to slide the decal about before pressing down.
GW decals don't need much trimming as each has its own carrier film which doesn't overlap the printing by much, Some other brands just print on a single film, so each decal does need to be trimmed individually.
Another tip - don't leave the decal in the water too long or you risk losing all the adhesive. Try to float them on the surface, don't submerge. I allow mine to get wet all over, then take them out and wait until the decal comes loose from the paper. One the backing paper is wet all the way through (goes darker), that should be enough soaking.
Get yourself some very fine-point tweezers for handling the decals once soaked.