Now engage in some progressive relaxation. Spread the relaxation through your entire body. While you are doing so, tell yourself that you are going deeper into hypnosis too. You can use any progressive relaxation method you know of, here are a couple of ways you might consider: 

a) You can simply breathe and say the word 'soften' to yourself as you think of the muscles of your body. Work your way through your body, using your awareness, starting at one end of your body and moving all the way through to the other end. This is a process I learned from Richard Bandler, one of the co-creators of the field of NLP (neuro linguistic programming) when attending a seminar of his.  

b) You can imagine a relaxing colour, and then spread that colour through your body, one muscle at a time. Tell yourself that as the colour spread, it relaxes the muscles. 

c) Imagine light and/or heat spreading through you, relaxing you deeply.

d) Imagine that you are a rag doll and that your muscles are loose, limp and dormant.

e) Imagine being close to a heat source that spreads throughout you.

f) Imagining tensing specific muscles as you inhale and then relaxing them as you exhale. Doing this systematically starting at one end of the body and finishing at the other end. 

There are many, many other ways to use progressive relaxation. Just use whatever process you know of to deeply relax and allow your body to be more and more relaxed. 

When you feel really relaxed, focus the relaxation on your stomach area. Get it so relaxed that it would be very hard, if not impossible to feel nausea. When you have your stomach relaxed deeply, move on to the next step, while maintaining that nausea.  

Imagine that you are now in that typical situation, that place where you used to feel nauseous, that you were mindful of prior to starting this session.  

Notice all the conditions being present, yet you remain relaxed and at ease. Notice that you are in this situation, yet you keep comfortable and at ease. 

Use your cognitions (tell yourself using internal dialogue) to believe in this, know it is going to happen, believe in it and tell yourself that this relaxed, comfortable response to this scenario is going to happen in the future. Convince yourself that it is going to be extremely difficult to vomit in this situation in the future... Even convince yourself that if you tried really hard to vomit, you'd really struggle to do so. 

When you have really convinced yourself in a gentle assured manner, move on to the next step. 

(n.b. If there is not such a typical target situation that triggers things for you, then you can skip this step, however I'd recommend that you perhaps use your imagination to mentally rehearse a time of some kid whereby you deal with it and are comfortable)

If you really believe that you have to experience nausea, there is no need to resist it or get anxious trying to resist it; that could be counter-productive. Therefore, permit yourself 10 minutes a day (you choose the time of day that you'll do this) when you are allowed to experience the nausea. Then over the space of the next week, reduce that by 2 minutes for a couple of days and then reduce it by one minute a day, so you experience the nausea for shorter periods of time.  Reduce the period of time gradually until it is non-existent in your day. 

(Again, you may choose to skip this step if you would rather be free of it and are sure that you can take control of your nausea as a result of the other steps.) 

You may have worries or concerns about vomiting that actually contribute to the nausea. Therefore, now is the time to practice dealing with any worries about vomiting. You can simply apply socratic questions if and when you worry about vomiting; resulting in helping you to realise how misplaced any worries are. 

Typically, these are questions along the lines of: 

What evidence do I have for (and against) this worry? 

Are the grounds for this worry 100% accurate and realistic?

Where is this worry getting me? What are the consequences of having this worry? 

How is this worry affecting my desired outcome?  

Are things the way I am really portraying them to myself? 

Would my loved ones agree with this worry? 

Is this reality or just one possible perception of the situation? 

There are many more, but I hope this gives you a flavor of looking for evidence, logic and reality by questioning any worry you may have about the nausea, worry that may have been contributing to it. It is common for worry to become a fear of being sick, which in reality, happens rarely for most of us; certainly not enough to have developed a fear of it. 

(You may not have such worries, in which case you may also choose to skip this step) 

In a nutshell, you now do this: Create a numbness in your hand and then spread it to your stomach area to be free of the nausea. Firstly though, you'll want to practice the anaesthesia and have it all set up to use accordingly. 

Here are a couple of articles to help you with that:

Using Self-Hypnosis To Create Numbness & Anaesthesia

http://www.adam-eason.com/2012/11/22/using-self-hypnosis-to-create-numbness-anaesthesia/ (with this article, I recommend using the option of turning the area into a 'woodenlike' sensation, which lends itself well when dealing with nausea)

Using Self-Hypnosis To Create Anaesthesia (this is a more simple glove anaesthesia write-up) 

http://www.adam-eason.com/2011/01/14/using-self-hypnosis-to-create-anaesthesia/ 

Once you have changed the sensation in your stomach and/or shown that you are in control of it, you can move on to the next step.

Take some time out to reflect upon what you have done, and then promote some of the key aspects of the session... Enjoy some more relaxation, enjoy the comfort you have, remind yourself of how things are going to be in the future, then move on to the final step. 

Take a couple of deep, energising breaths, wiggle your finger and toes and open your eyes to exit. 

Practice this process in hypnosis every day for a week to develop control and build efficacy with the skill. Then put yourself in those target situations and practice being in control and being free of nausea. 

Jacknow, D. S., Tschann, J. M., Link, M. P. & Boyce, W. T. (1994) Hypnosis in the prevention of chemotherapy-related nausea and vomiting in children: a prospective study. Journal of Development and Behavioral Pediatrics, 15: 258-264. 

Kroger, W. S. (1977) Clinical and Experimental Hypnosis (2nd ed). Lippincott, Williams and Wilkins, PA. 

Syralja, K. L., Cummings, C. & Donaldson, G. W. (1992) Hypnosis or cognitive behavioral training for the reduction of pain and nausea during cancer treatment: A controlled clinical trial. Pain, 48: 137-146. 

Zeltzer, L., Kellerman, J, Ellenberg, L. & Dash, J. (1983) Hypnosis for reduction of vomiting associated with chemotherapy and disease in adolescents with cancer. Journal of Adolescent Health Care, 4: 77-84. 

Zeltzer, L., LeBaron, S. & Zeltzer, P. M. (1984) The effectiveness of behavioral intervention for reduction of nausea and vomiting in children and adolescents receiving chemotherapy. Journal of Clinical Oncology, 2: 683-690. 

Zeltzer, L. K., Dolgin, M. J., LeBaron, S. & leBaron, C. (1991) A randomized, controlled study of behavioral intervention for chemotherapy distress in children with cancer. Pediatrics, 88: 34-42.