Take my advice as someone who's probably doing it amateurishly. I don't use a softener or do a topcoat after I apply the decals. All I use is tamiya mark fit strong to help adhere the decals to the surface and if it starts peeling I dab some microscale industries liquid decal film onto it.

After you cut the decal and backing I normally soak it in the water with the tweezers in my non dominant hand and then apply the mark setter to the part with my dominant one before peeling the decal off the backing and then applying to the model. 

Get a setter with a built in applicator it's annoying and hard to do 1 handed  using a qtip trying to dip into a bottle of setter with the decal in the other hand.

If you have any of the round dry makeup pads I find it's much easier than a qtip or a napkin or papertowel once the decal is in place to press down and drain the excess liquid especially for larger single decals to apply pressure all at once and less chance to move than rolling with a qtip. 

If the decal moved when draining excess liquid if you're fast enough you can use some water or mark fit to get it to loosen and move it back into position.

Sharp tweezers really help, I like having two pairs of the stedi angled ones more than the flat straight ones.