I have lost significant weight and now have loose skin. How can I tighten up my skin?

It will take some time for your skin to tighten up, as it is depends on genetics, your age, and how much weight you have lost.

It is commonly recommended to drink lots of water, and to apply something like cocoa butter or coconut oil onto the loose skin. DMAE has been found to help tighten up skin[1][2][3] and could also be useful.