Take your time, it always takes longer than you think so dont rush it as it looks like youve made a really good job of the paint and masking, even if you take a few days to finish it.
I just use scissors to trim just slightly around the outside of the printed decal and make sure all cuts around corners have curved smooth cuts, not sharp corners which can lift or catch on things over time. Have a bowl of warm water with some fairy liquid in, submerge the stickers for a few seconds then apply to the shell and gently stretch them around the curves. If they are still wet you can repostion them and when in place flatten down with a paper towel while squeezing out any bubbles. If I need the decal to shrink into a corner or around a tight bend I give it a quick waft over with a ciggy lighter. Any bubbles can later be popped with a bin to release any trapped air/water under the sticker.
Other than that; make sure you peel the film off first and wash your hands, stickers dont stick to well to shells covered in greasy fingerprints and if you inadvertantly get the sticker stuck on your finger and it has a transparent backing you will see your fingerprint in the decal on the shell forever after.