Learning self-defense all by yourself isn't optimal -- but if training by yourself is the only option, it's much better than doing nothing at all. Don't worry about practicing joint locks or other complex techniques; the only way to master those is with hands-on practice. Instead, focus on practicing strikes to vulnerable areas of the body, improving your situational awareness and building a resilient self-defense mindset.
Practice Hand Strikes
To defend yourself, you don't want to trade blows with your opponent or try to execute fancy joint locks that require fine motor control -- you want to put him down as quickly and decisively as possible. That means using strikes to vulnerable parts of the body, including the throat and eyes. Practice your hand strikes using X-ray paper, kicking paddles or a hanging speed bag as a target. Practice striking with your elbows, too. Upright punching bags shaped like a human bust are also useful for practicing your visual targeting, although they might not be tall enough to simulate the positioning of an actual attacker.
Kicks to the knee or the groin, as well as knees to the groin, provide effective ways to quickly inflict major damage on an opponent. The ideal practice target is a hanging punching bag that you can adjust low enough to simulate knee or groin height, or a large, rectangular kicking shield suspended between a cable or bungee from the ceiling and another from the floor. In a pinch, you can also fold a couple of blue gymnastics mats and place them against a wall as a makeshift target for kicks, but don't try to knee that target -- it doesn't have enough give.
Build Your Mindset
For some people -- women in particular -- one of the hardest parts of defending yourself is simply giving yourself permission to inflict serious damage on another human being if it becomes necessary. Getting used to the idea that you can -- and should -- use whatever force necessary to defend yourself is something you can do on your own. You can help get yourself comfortable with this idea by following through, using full force, on whatever hand, elbow, knee or kicking strikes you're practicing. Imagine that you are jabbing, kicking or elbowing all the way through the target instead of just tapping it, because in an emergency you will respond in exactly the way you've trained.
Make a Scene
Another aspect of self-defense that people of both sexes -- but women in particular -- struggle with is being able to set loud, forceful boundaries; one way attackers get close is by persistently offering help or company and counting on your not being willing to make a scene by turning them down. Often, the mere act of making a scene is enough to drive an attacker away and bring help on the run. Practice vocalizing a loud and forceful "no," or whatever else you want to shout to make a scene, while looking in a mirror or at an object or picture that you've chosen to represent an attacker.
Become a Strategic Thinker
Situational awareness is a big part of successful self-defense. Practice it on your own by staying aware of people around you; parking and walking in well-lit, well-populated areas whenever possible; and looking in the backseat and underneath your car as you approach it. More than anything else, get used to the idea of trusting your gut. If it tells you something seems off, it's probably right.