here is a guide. I also followed another guide but forgot the correct search parameters to have it be seen in google but the link below has covered the important points
http://otakuonabudget.blogspot.com/2014/05...ide-decals.htmlThe article is from PH though and we can easily buy decal papers in our bookstore, I am sure you have some equivalent store there in MY.
I dont do vectoring or imaging application, I just grab PNG pictures on the net and resize in Microsoft word (you can use excel, power point as well, as long as you are not making your own design), test print using normal papers and see if it is okay. Once I am done, I print using Decal paper (must use the correct paper type for the printer you use) then after it is dry, I put a layer of topcoat (5 to 6 mist spray using Bosny)
I mainly use CLEAR decals since it is easy to apply, just cut round around the decals and apply, main dis-advantage is, it is not opaque and any WHITE color will be clear.
White Paper decals can print opaque decals (meaning no color bleeding from the color of the plastic or paint) but you need to cut the decals exactly on the shape of the decals.
I dont like the most 3rd party Decal products from samuel decals and taobao (basically source of samuel).
Here is a sample from my work.

DIY decals are the ff:
Medusa skull (from one piece)
NUNS logo below the canopy
small skull logo behind