Now, to answer your question, it isn't hard if you know how to do it right. I heard a story of someone who used their toothbrush to make themselves throw up, but never use a toothbrush to provoke vomiting, because just as easily as it causes a gagging reflex, it can cause a swallowing reflex, and can cause you to swallow the toothbrush whole.
Warm salt water or ipecac can both make you throw up. Salt water can cause the runs if it eventually gets to the intestines, but if you throw up, the salt water will be evacuated.
So, as stated above, it isn't hard if you know the right techniques, but overall, I wouldn't make yourself throw up in the first place. If you are having stomach problems, or feel sick, please, take a trip to the doctor, or get some mental help if you are throwing up to lose weight.