What's the best way to improve foreign language skills online?

For Online learning, apart from Skype tutor (if there is a need), you should read and listen to news in the certain language. initially you might not be able to understand the word they are saying; write down the words or take down the word as you hear or read it. at the end of the day , you will have number of words meanings to which you will not be knowing. Only thing you will be knowing is their almost pronunciation. and then by the end of the day, look for their meaning. 

You need to increase vocabulary first, as much as you can and then go into grammar. major part of almost native speaker is to have a excellent accent with in depth knowledge of phrases and vocabulary and then work on grammar. if there is anything else i will be able to help you, it will just be a pleasure. 

Keep in mind, language is just a language, it is not a knowledge and almost every third language has something similar with the first one.

All the best.