How to import Leather private key?

First you have to create a NinjaWallet

Then you'll have to extract your private key first. Go to ADD ADDRESS on ADDRESSES tab and select Import private key as segwit then create.

Extract private key tutorial from Mikeinspace https://medium.com/@mikeinspace/bitcoin-stamps-importing-hiro-wallet-private-key-into-freewallet-desktop-app-c67745215423