Hi I was once oiver on the aveirah of משכב זכר hashem yeracheim. so my question is how do I do real teshuvah? and when I do real teshuva, will I get speared from the עונש of כרת?
The way to do real teshuva is by doing the regular teshuva. First of all to regret what you did, and then to say viduy. Meaning that you should speak to Hashem and say that you sinned to Him and you regret what you did. After that you should do something that will prevent you from falling into this Aveiro again. Think about what caused you to get into that situation, and take step to distance yourself from it in the future. This is all the standard teshuva. My understanding of your question though is that you understand that since this is a very serious sin, which need a higher level of kapara, you want to know what else you have to do. There are different things suggested to atone for such a sin, however the best thing that you can do is to increase the amount of torah that you learn. The kabbalah seforim say that the torah is compared to a fire, and when a person learns torah, or better yet toils in torah, this spiritual fire cleans the persons soul of all of his aveiros. (Understandably there are many levels to this, and the more the person learns, the better he gets cleaned.) This is especially true if the learning is done even when it is hard, and especially if the person sweats from his toiling in torah. This is what is suggested by the mekubalim, for us, especially due to the fact that we are weak nowadays, and by fasting, and paining oneself, it will cause the person to get weak and cause him to learn even less.
If you need suggestions ow to increase your torah learning, let me know a little about yourself, and I can give you some suggestions.
You should merit to do a complete teshuva and reap all the rewards that come to a person that merits to do so.
Reishis Chochma (Shar Hakedusha chap.13 para. 19), Ben Ish Chai 2nd year, beginning of Parshas Tetzaveh, also see Sharei Teshuva 4-16 who says this.