It was difficult at first. The first time I tried I walked up to a window in my LD and reminded myself that it didn't exist, then tried to run through it. I only succeeded in being knocked down So I took it a bit slower. I pressed my thumb up against the pane and pressed, constantly reminding myself that it didn't exist. Slowly, my thumb went through, and then my whole hand did! After that my whole arm moved in and out of it with ease. By my next lucid dream, I could move through anything effortlessly...because now I expected myself to be able to.