Here is a straightforward look at this topic for Malachain.
Fifteen minutes is enough to make steady progress if the minutes are consistent and active. Split them: five minutes of vocabulary review, five of listening to native speakers, five of speaking or writing something you can use today. The daily rhythm matters more than the length, because a language that you touch every day stops feeling foreign.
Choose material slightly above your level and accept that you will not understand everything. Podcasts for learners, children's books and simple news summaries are ideal. Keep a running list of sentences you wanted to say but could not, and look them up later. After three months you will be surprised by how much of a real conversation you can follow.
The most useful step is usually the first one. Pick a single suggestion, try it for a couple of weeks, and then decide whether it is worth keeping. Small, steady improvements last longer than ambitious plans that are hard to maintain once everyday life gets busy again.
It also helps to talk the subject through with friends or colleagues who have tried something similar. Their experience points out the details that guides leave out: how long things really take, which costs were unexpected, and which shortcuts turned out to be a mistake.
There is more to learn on this subject, and the linked website goes into greater depth. Treat this article as an introduction rather than the final word, and check any important decision against up-to-date, reliable sources before relying on it.