Here is a little tutorial (and a sneak of my next layout)...
STEP 1 - I started off by painting the chipboard bricks with sunburnt orange and bisque Dusty Attic paint - trying to avoid the casing so they wouldn't get stuck in there. Then, I added some dark brown ink on top.
STEP 2 - I used snow writer (by Deco Art) and randomly semeared it on - again trying to avoid the casing so they wouldn't get stuck in there.
STEP 3 - I went over all of it with light brown ink to blend the colours.
STEP 4 - I whitewashed it all with watered down gesso to give it a soft aged effect to suit my whitewashed project. NOTE: Omit this step if you are using on a darker layout.
STEP 5 - I added some of the bricks to my project and filled the spaces with the snow writer (easy to do since it has a really fine tip). Alternatively, you could slather on any paste of your choice and then set the bricks into it (but it's not a precise application that way). TIP: Use a toothpick to flatten in the groves a bit.
I will be back soon with the new layout and a challenge/contest (with a PRIZE of course :)