Decal application: clean the area to be applied to with an anti-grease agent and wipe over with soft dry cloth (if not, a glass cleaner at the very least!) Allow a bit of extra time for the frame to air dry (evaporate). Peel the opaque tape away from the white backing (making certain that all the decal is attached), then show the decal up to the frame for alignment. Without pressing, make sure you are happy with your position, then apply the middle of the decal only (like on the down tube, so that the upper and lower parts are not touching. At this point you should still be able to make any adjustments. When happy with position, gently apply the upper section, then the lower removing any bubbles or wrinkles as you go. Rub the decal through the opaque tape, then gently remove the tape, leaving the decal in place. Some sites advise using a squeegee, but for an area as small as a bike frame, not really practical.
Depending on the decal, it is not always necessary to lacquer over, except when the lettering/design is really fine, or when the decals are digitally printed (not totally necessary, but will increase life span). Of course, it is always a more pro job to lacquer over, but if you are doing a 'DIY' job, not always an option. On my frames, I use an 'air dry' spray obtainable from many car accessory shops or DIY centres such as Halford's or B&Q, but only ever use a quick flash over as not to excite the paintwork! I have one bike which has had un-lacquered decals for almost 14 years. It's looking a bit rough now, but any bike would be after 14 UK winters with washes after every ride!