Sri K. Pattabhi Jois, the founder of ashtanga yoga, was a man of very limited English. When trying to explain mula bandha to his American students, he would simply say, "Tighten your anus." It is slightly more complicated than that, but that is the gist of what we are trying to accomplish with mula bandha. We are trying to close off the lower orifices to prevent energy from leaking out of the body. Mula bandha also has a lot of effect on posture. With this bandha engaged, you will naturally align your lower spine which will in turn help you find greater comfort and freedom in your yoga postures.
Sit comfortably with your legs crossed. Take a look at your knees. You should place a pillow beneath your tailbone if your knees are higher than your hips. Keep adding blocks and pillows until your hips are elevated above your knees.
Push your tailbone out behind you and sit up tall. There should be a slight curve in your lower back. Let your hands rest comfortably on your legs. Either close your eyes or focus them on the tip of your nose. Breathe consciously in and out. It doesn't have to be a deep or forced breath, just fill the lungs and let it go. We are calming our minds and focusing on the task at hand, letting everything else fall away.
When you feel comfortable, we will move on to the next piece - something yogis describe as "lifting the pelvic floor." You have probably never done this before in your life - and there really would be no reason to if not for mula bandha - so don't worry if the muscles don't respond right away. What you are trying to do is find the muscles between the anus and the urethra and hug them up closer to the body. Again, take a couple breaths here and get used to using these muscles. Make sure you are also still closing off the anus.
At this point, your course of action will differ if you are male or female. If you are male, you will try to pull the testicles up and closer to your body. If you are female, you will spiral the walls of your vagina closed. If you are having trouble making the muscles obey your commands, you can try the following trick. Close your eyes and try to picture the muscles you are focusing on. As you hold these muscles in your mind, ask them to contract. Don't be discouraged if this is difficult, it may take some time before these muscles respond to your commands. Keep breathing. Try to hold all of these different muscles effortlessly.
When you have take a few breaths and are comfortable moving on, the last piece of mula bandha is to close off the urethra. Think of the last time you really had to pee and you were trying to hold it. Again, we're trying not to clench, but just persuade the surrounding muscles to close.
And that's it. Mula bandha. Take a short rest and then repeat the procedure. Tighten the anus, lift the pelvic floor, hug the testicles in, spiral the walls of the vagina closed, close off the urethra. With sustained practice you will be able to move into this bandha more quickly, and hold it more effortlessly.
Let's try this bandha while in another yoga pose. Step into Warrior B: feet planted, hips facing the side, pelvis tucked under, arms parallel with the ground, shoulders relaxed. Gaze over your front middle finger and let your breathing be calm and steady. Now, we will add in mula bandha. Tighten the anus, lift the pelvic floor, hug the testicles in, spiral the walls of the vagina closed, close of the urethra. Feel the greater support through the pelvis in this pose. Notice the automatic alignment of your lower spine.
The idea is to hold mula bandha throughout the entire practice, in all of your poses. It is hard to remember at first, especially when you are also focused on breathing, balance, drishti, and the pose itself. But with enough practice, the bandhas will become second nature and you will do them naturally without having to remind yourself. As Pattabhi Jois used to say, "Practice. All is coming."