You'll need Java JRE for this and AVABox Grab the jar or apk you want to play with, put it in avaboxroot/smali/baksmali. Use 7-zip to extract classes.dex. Pull up cmd.exe as an administrator and change to that directory. Then:Do your editing, then, to repack it, typePut it back into the apk or jar and run with it.
java -jar baksmali-<version>.jar -o classout/ classes.dex
java -Xmx512M -jar smali.jar classout/ -o new-classes.dex