In Photoshop, by overlaying the texture patterns on the colored parts. On the paint bucket tool, please use a drop down box and change from Foreground to Pattern. You can make your image into a pattern by going to Edit → Define Pattern.
From the selected area, use the paint bucket tool with the chosen pattern then change the layer option to → overlay → opacity 30%. I usually use 30% opacity, but it can be adjusted to any percentage you want.
Very beautiful picture. Nice colors, I admit, I dont care much about other Vocaloids other than Rin and Len.... And I only like Rin x Len pairings.... T-T...... NO KAITO X LEN!!! Anyways, beautiful pic and AMAZING DETAIL<3 Then again, I suck at drawing. But this... Wonderful.