This is easy.. If you know how...
Right here is a checklist

...
- Get in a decent gym
- Figure out the best time to go & learn to use the equipment
Once you have learned about training and when to train your good to go: with the following...
- Research about workout routines (For muscle mass i would suggest 3sets of 5-6reps.. Aim to not be able to complete any more other than your last rep)
- Research about diet (Yes you do need to diet, keep your diet clean to get lean growth.. its 100% diet and 100% training..)
Then all you have to do is stick to your routine
Monday - Chest & Tri
Tuesday - Legs
Wed - Rest
Thursday - Back & Bi
Friday - Shoulders & Traps
Sat - Cardio
Sun - Rest
Also you should aim to eat 5 - 6 clean meals a day, and also use supplements like:
- Whey protien
- NO2 (insane pumps)
- Creatine Monohydrate
- Amino acids & multi vits.
Read everything you can todo with workouts; 1 workout works for different people.