*download to read. ----- There are many ways to add texture but this way is, by far, my favorite 8D this is also the most simple way to do it. I hope you can understand this tutorial, since I'm bad at explaining with words, let alone in English.
you can get free texture/pattern from many sites such as Bitbox, CGtexture, even here on dA. the ones I used are from Pixiv.
if you have question regarding this tutorial, feel free to note me. I will reply as soon as I can
more tips: if you want to add the whole image of the texture, just press ctrl + A before you copy the texture. you can rotate, flip, and even adjust the hue of the texture as it becomes a new layer.
I'm a slow learner for SAI... I just tested out some skills on my own tonight... I still suck but so far SAI is better for a few things. I just need to be more quick on the shading being I am more used to Adobe short cuts & know how there ^^: