Understand the difference between marijuana and hemp. Hemp and marijuana are both varieties of the Cannabis sativa plant. They both contain cannabinoids, a class of chemicals that activate cell membrane receptors. The most well known of these cannabinoids is tetrahydrocannabinol or THC. The marijuana plant is the female plant; the hemp is the male plant. You need the female plant to make effective medical marijuana oil.
Know that medical marijuana oil is not the same as hemp seed oil. Hemp seed oil is a cold-pressed oil made from the seeds of the hemp plant. It's rich with essential fatty oils and used for its nutritional benefits. You can easily buy it in health food stores. It often gets mislabeled as "hemp oil," but it's not. True hemp oil (aka medical marijuana oil), on the other hand, is made from the buds of the female hemp plant. The THC in the plan allows it to impact cells and offer medicinal benefits.
Embrace synonyms. Medical marijuana oil is also known as hemp oil, cannabis oil or hash oil.
Making Medical Marijuana Oil
Purchase and dry your plants. Buy the best quality plants you can afford. The better the quality, the better the oil. Make sure your plant buds are as dry as possible before beginning. Because this process is rather involved, you may want to start with a large amount--a pound (454 grams)or more--however you can use as little as one ounce. The amount of oil you get will vary from strain to strain; an ounce will usually produce 3 or 4 grams of oil.
When you buy your plants, get a strain that has the highest possible THC content. A good Sativa strain is recommended if you want to energize someone suffering from depression. For most other medical conditions, the suggestion is to try an Indica strain, which helps with relaxation and promotes rest and sleep. 
Decide on a solvent. You'll use the solvent to strip the THC off the plant. You'll need two gallons (7.57 liters) of solvent for one pound of dry hemp. About 17 ounces (500 milliliters) of solvent should be more than enough if you're working with one ounce of starting material. There are a variety of different solvents you might try, but two of the most common are alcohol and naphtha.
Pure naphtha is inexpensive and, since it is used as a paint thinner, readily available at most paint supply stores. In Europe, you may find it under the name "benzine."
For a solvent to 100% effective it should be 100% pure, so look for pure alcohol--it can be expensive and more difficult to find.
All these solvents including alcohol are poisonous in nature, so it's imperative that you follow instruction for making hemp oil very carefully.
Stay away from ether and butane. Although ether works extremely well in this process, it's very volatile and is best used under laboratory conditions. Butane is likewise very volatile and produces a less-effective product. 
Do some additional research to learn what other solvents people have used successfully.
Do your first wash. Put the dry plant buds in a large plastic bucket and add your solvent. Use a stick of clean, untreated wood to crush the buds and leaves. Then add more solvent until the plant material is completely immersed and continue crushing for about three minutes. You're working to dissolve the THC off the plant material and into the solvent.
It is very important that this process be done in a well-ventilated area away from cigarettes, flames or sparks because the solvent is extremely flammable.
Pour the solvent-oil mix off the plant material into another bucket. Leave the hemp in the starting container. At this stage, you've already stripped the plant material of about 80% of its THC.
Do a second wash. Again add solvent to the plant material and work it for another 3 minutes to get whatever beneficial resin remains. When you're done, add this solvent-oil mix to the bucket you used to pour off the first mix and discard the plant material.
Filter your oil. Get some empty water bottles or gallon jugs and insert a funnel into the neck of the bottle. Put a coffee filter in the funnel and pour the solvent-oil mix through to remove any unwanted plant material.
Boil off the solvent. You need to heat your mixture so that the solvent boils away and just the oil remains. Fill a rice cooker until it's about three-quarters full and put the heat setting on "high." Keep an eye on the mixture as it cooks. As the level of the solvent-oil mix in the rice cooker drops, keep adding more until you have none left.
It is very important once again that this process be done in a well-ventilated area away from cigarettes, flames or sparks because the fumes are very flammable. Set up a fan to blow the fumes away.
A rice cooker works really well for this step of the process because it has a built-in heat sensor that prevents rice from burning. In this case, it will prevent the temperature of your solvent-oil mixture from becoming too high. If it does, the cannabinoids will vaporize off of the oil.  At no time should the temperature of the oil go over 290 degrees Fahrenheit (140 C).
Remove the last of the solvent. When there is about 2" of solvent-oil mix left in the rice cooker, you need to add a few drops of water to boil off the last remaining amount of solvent. If you started with a pound of plant material, add 10-12 drops of water. If you started with an ounce, you will need just a couple of drops. When the solvent has been boiled off, the rice cooker should automatically switch to the "low" setting.
If you'd like, you can put on a pair of oven mitts, pick up the rice cooker and swirl it to aid in evaporation.
The water will cause the oil to bubble and crackle, and you'll see a little bit of smoke or steam coming off the oil, but that's completely normal.
Vaporize the water. Put on your oven mitts and remove the inner pot containing the oil from the rice cooker. Gently pour the oil into a small stainless steel container. Place this container in a dehydrator or put it on a gentle heating device such as a coffee warmer. When the oil stops bubbling, remove the container from the heat and allow it to cool a bit.
Depending on the strain of plant material you used, the water could boil away quickly, or it may take a few hours for the oil to stop bubbling.
Package the oil for use. Immerse the tip of a plastic syringe into the oil and gently pull back on the plunger to draw the warm oil up into the syringe and let it cool. You can simply pour the oil into a clean bottle rather than use syringes, but the syringes do make it easy to dispense the medicine. Store the oil in a cool dark place for maximum shelf life.
The oil will become a thick grease, which could make it difficult to force of the syringes when cooled. Just run hot water over the syringe, and the oil will dispense much more easily.