I think you should challenge yourself with a Big Project.
Use your sketchbook to first lay out a Scene,possibly one of action,maybe one of a quiet moment,just expect it to be complex witha LOT of details. But do this layout Very Roughly,no more than vague blobs and forms of where these details will go.
The next step is to work out the details in your sketchbooks. Each element of a scene should add to the "story" of the whole: the setting is signifigant,the costume of the figures,the objects they are holding,and what's going on,can all tell a story and expand your illustration. A Maiden sitting on a windowsill drinking wine,looking out the window. Is she Rich or Poor? Dressed finely in a ballgown,or getting ready for bed? What is the wine in,a fluted goblet of crystal,a gold chalise,or a servicable earthenware cup? What time of day is it? What does she see outside? A Scene unto itself,or a glorious landscape,backround to her own thoughts? Just a few examples,and the details will come slowly but will both inspire and instruct you in the creation of them.
Once you decide what sort of situation you want to draw,then think about the composition. Interesting stuff overlapping the main action will add depth to the illustration. Foreground bits can be as important as background. In fact,after you design a bunch of details,I'd go back and tweak the layout and incorporate these new elements. Eventually,you'll star on the Big Picture,done on the BEST materials you can find. Lay in the background very lightly,then build the scene outwards,then once the composition is in place,make the lines darker and with patience you'll have a Masterwork you'll be proud of.
This pic is one of mine,and done with scanned bits and assembled in photoshop. It had over 90 ayers to it,not counting details preassembled from their own bits like the toolcases and shelves. If you can use Gimp or similar,go for it,but anyone can do this with lots of planning and time.
