You can't clone a leaf unless you are good at tissue culture...
The last chance to clone is about 2 weeks into flower, and even that is a bit shaky, especially for a noob. Your bet bet is to search the term 're-vegging' and follow the instructions. Hint: Post-harvest, leave leaves on, and low bud sites, use 24h blue light, water LIGHTLY, and add a bit of superthrive to the water.