1) Make sure your shoulders are sufficiently rehabbed to allow for strength training. If you haven't already, consult a sports physiotherapist to assess your ability to safely perform resistance exercises. The exercises you want to be able to do are squats, deadlifts, bench press, overhead press, chin/pull ups (or lat pull downs) and rows. Until you can safely do these exercises with barbells or dumbells (NB: standard squats and deadlifts should always be done with barbells as dumbells make for inefficient loading on these exercises), perform bodyweight squats, bodyweight lunges, and upper body push/pull and rehab exercises as prescribed by your physio.
2) When you are physically able, do the following:
A: Squats, Bench Press, Pull Ups/Downs, Prehab exercises (exercises prescribed by your physio to maintain shoulder stability and reduce risk of re-injury; these will typically include some rotator cuff work and possibly some stretches).
B: Deadlifts, Overhead Press, Rows, Prehab exercises.
Train 3 non-consecutive days per week. Day 1 is workout A, Day 2 is workout B, Day 3 is A, Day 4 (the first training day of the following week) is B, and so on.
On the first three exercises of each day, start with a light-to-moderate weight that you can comfortably perform 3 sets of 6 reps with (after 2 lighter warm up sets, 1 at 50% working weight and 1 at 75% working weight), without invoking shoulder issues. Once you get 3x6, next time you do those exercises aim for 3x7 at the same weight. Once you achieve 3x7, the next time you do that exercise will be aiming for 3x8. When you complete 3x8, increase the weight by 5-15lb and return to 3x6, starting the cycle all over. Warm up sets are always for 6 reps. Perform prehab exercises as prescribed by the physio.
3) Nutrition! Figure out how many total calories you typically consume in a day at the moment. This is most likely your maintenance level. Reduce this number by 500, and try to stick to that amount of total calories daily. Within that amount, consume 180-200g protein/day (720-800kcal worth). It's quite simply a matter of consuming less calories than your body needs to maintain weight, while consuming enough protein (snd training appropriately) to preserve (and potentially build) muscle mass while in this weight loss environment.