“I'm just putting the finishing touches on it.”
You like baking fancy cakes. You've made a cake for your nephew's birthday. Now you're decorating it. You say this to your husband when he comes into the kitchen.
I'm just putting the finishing touches on it.
You can use "I'm ___ing" to talk about things that are in the middle of happening now:
I'm waiting for the bus.
...or about things that are going to happen soon in the future:
I'm moving to Sweden in March.
People usually talk about the future this way when they're discussing plans that have already been set or chosen.
When you make something fancy or beautiful, like a cake or a painting, the details are very important. The “finishing touches” are the last details that make something truly complete. We use the phrase “put the finishing touches on ___”:
I just need to put the finishing touches on my presentation for tomorrow, and then I’ll be ready.
I’m going to wait to put the finishing touches on my makeup, so it will be fresh when I arrive at the party.
You say this when you are doing something small, and will be finished soon.
I’m just putting my shoes on.
I’m just checking my messages.
I’m just reviewing the last section.