Practice with a .22 rimfire. It is really cheap to shoot and will improve your overall accuracy. Practice shooting on a bench first that is where you can practice your breathing techniques. Once you get that down practice offhand shooting along with your breathing. After that practice pratice and practice some more. That is a technique for helping you to be more confident with your shooting abilites. As for calming down when a deer suddenly appears in your sight, good luck. No, what I try (stress the word try) to do when I see a deer is take a few deep breaths and concentrate on putting the bullet on a specific spot. Hopefully all the practicing will come into play and you will naturally breath correctly and squeeze the trigger. Hopefully! Practice and experience are your two major factors in clean kills on game. Good luck.