I started falling in love with language when I decided to learn Latin to help further me in the medical field. My school didn't offer Latin, though, so I had to take a class online. When you take a class online, you're basically teaching yourself with someone available to call if you really need it - but only during business hours. For me, it was an intro on how to teach myself a language. I now know six languages and am teaching myself my seventh - only two of which I didn't teach myself. Teaching yourself a new language is a lot of work and takes commitment and continuous motivation, but it's not impossible.
1.) Pick A Language
The first step may seem like the easiest, but it is actually really important and effects the rest of your learning experience. When picking a language to learn you need to ask yourself why you want/need to learn that language. Wanting to learn a new language for fun is a perfectly acceptable answer, but if this is your only reason it's going to take strong motivation.
2.) Gather Materials
You should get your hands on as much learning material as possible. I use textbooks I've found online and printed out, phone apps (Duolingo is really good), and videos of people that can go over lessons (usually on YouTube). You will also need to use material you might not think is exactly academic: watch movies and music videos in that language with the subtitles on. This gets you used to dialect and you can actually pick up on some words and phrases pretty quick.
3.) Find Motivation
Motivation is really important to learning a new language. It can be because you want to visit a country, because you are romantically involved with someone who's first language isn't the same as yours, or because your family hails from that country and the language just didn't trickle down to you. Whatever your motivation, make sure it is something that is going to keep you going; learning a language is a long and hard process and it will be easier to give up than to keep going during those times you feel like you're getting no where.
4.) Map Out Your Path
It's always a good idea to plan what you're going to learn when. It's a good idea to block what you learn: greetings first, then introductions, colors, numbers, food, etc. Don't try to learn things that aren't in the same group at the same time, it will make it a lot harder to remember the vocabulary. You should always start with conversational vocabulary (words and phrases that can help you get around or make small talk).
5.) Set Goals
You should set daily, weekly, and monthly goals. How many words do you want to learn in a week? Do you have the grammar down yet? How good is your accent? All of these things are important steps to learning a new language and you should set goals for yourself so you don't get off track and fall behind.
-- Accent Note: You should try to get the accent down as much as possible, however, if a native speaker notices the language isn't your first because you're accent isn't perfect, they're more likely to forgive grammar mistakes.
6.) Make Your Curriculum
The biggest struggle with teaching yourself a new language is the urge to cheat. You need to be hard on yourself, but don't set yourself up for failure. Make flashcards, create worksheets, and make quizzes and tests as you learn.
Make flashcards for each word you use but also phrases. It's best to put the new language in the middle of the white side of the card -- your brain will retain information on a white background. Mix up whether you show yourself the new language or your first language so that your brain makes the connection.
Worksheets should be challenging but not something you're going to want to rip up as you cry. Write out sentences with a word or two missing for you to fill in, generic conversations that you have to respond to, or something as easy as translating words or phrases. Your worksheets should challenge your knowledge of the vocabulary, grammar, and spelling. Don't attempt to complete the worksheet right after you make it, give yourself a period between study time and the worksheet so you know what you have actually retained and what you need to work on. A fun idea for worksheets is to think like a kid; if you're learning parts of the body, draw a monster and label!
6.3) Quizzes and Tests
With the quizzes and tests, just like the worksheets, add vocabulary and grammar as you learn and save them for later. It's a really bad idea to make a quiz and then take it immediately after, it's basically cheating and you won't know what you have actually retained. Quizzes should be based off the worksheets, but not their twins. Have the same vocabulary used from the worksheet, but mix it up a little and make it harder. Tests should be based off a few of the "blocks" you've been using; basically, just like in a regular class, a test should be an accumulation of a few quizzes.
7.) Practice Makes Perfect
Don't practice until you get it right, practice until you can't get it wrong. Use your vocabulary every chance you get. When greeting someone, greet them in the language you're learning, then translate. When getting something from the fridge, name the foods that you know (write shopping lists in the new language). While driving down the road, say the colors of the cars you're passing. Doing little exercises like this will not only help you practice, but also make real-world connections with the new language.
8.) Find A Friend
This isn't necessary, but having conversations regularly with someone who is fluent in the language will help you get practice. Having a friend who is fluent is also convenient when you have questions that you can't exactly answer yourself (for example, the difference between Guten Nocht and Gute Nocht). Just don't rely on them to teach you the entire language -- not that they wouldn't, it's just kinda rude.
Learning a different language is not only learning different words for the same things, but learning another way to think about things (Flora Lewis). I hope this helps on your journey to teaching yourself new languages. Remember that it is a difficult road, but it's not impossible! If you have any questions or need help making any curriculum, feel free to comment. Good luck, don't forget to have fun!