Wendler put agood approach to this in one of his recent spills. Barbell presses, dumbbell side laterals, rear delt raises in a circuit for a number of rounds. Do this a few times a week. On the in between days, do different shoulder movements- turkish getups, seated DB cleans, Arnold presses, stuff like that. Keep the weights lighter but the volume high. Don't be afraid to hit them several times a week. Don't go to failure, but approximate it.
In compound movements, focus on the movement. In isolation movements, focus on the working muscle. For instance, while you may be able to do side laterals with 30lb dumbbells, you're likely activating your traps to do so. So lighten up the weight and focus on keeping traps out of the movement.
First and foremost you need to stabilize your body weight. If it continues to decline, you're not going to miraculously get bigger shoulders. Eat more. When you're absolutely stuffed, wait a few minutes, then eat more.
This topic has been covered a hundred times over on this site. Pick an approach, stick with it for 12 weeks without changing it, pay attention to food consumption, evaluate the results.