Although I am a non-vegetarian, I have helped countless vegetarians gain muscle weight.
I want to point out to the fact that getting protein for vegetarians is not tough and you can cross 100gms daily (if it is required). You can read about it here.
Now, the first step is figure out how many calories one need to consume in order to maintain weight. This is also termed as the TDEE (Total daily energy expenditure).
TDEE is the amount of calories you burn in a 24 hour period. Its the sum of your basic metabolic rate and thermic effect of calories. There are 2 accurate formulas which can figure this out for you:
1) The Katch-Mc Addle Equation
2) The Mifflin-St Jeor Equation.
For more details please read this.
Once you have figured out how many calories is required to maintain your weight, you need to eat roughly 15 to 20% more than this to gain weight and build muscle.
Eat more than this percentage (i.e. more than 25% of your TDEE) you risk gaining more of fat, and eat less ( less than your TDEE ) you will not gain muscle.
To get your macros (carbs, protein and fat) right, is a hit and trial process and you will get better with time.
Now, calorie counting is a rather tedious process, instead do the following things:
1) Note that we all eat very limited variety of meals. For example, I have been eating eggs in my breakfast, for over a decade now, and I am still not bored!
Anyways my point being our ingredients list is very limited and once you start “googling” their caloric values and maintain a dairy for the same. Over time, you will have a pretty clear idea of how much calories you are consuming.
2) Eat slowly. It takes our brain 15 minutes to figure out whether we are full or not. so once you are full, eat slightly more. Only slightly.
Here is a sample vegetarian diet plan to build muscle:
1) 4 Whole eggs+ 2 whole wheat bread with a glass of cows milk, add oatmeal as an option.
You can also make your oatmeal with milk to increase the protein content.
2) Indian thali: 3 rotis+ 2 bowls dal ( 17gms protein roughly ), a big bowl of salad and mix veg. Try and get it cooked in desi ghee. Why?, its a myth that ghee is bad for your health. Find out here.
3) Add snacks: Almonds, walnuts are the best. Eat bananas generously as pre workout and post workout.
4) Postworkout: A big glass of banana shake, add more fruits like strawberries etc if you like. Add a little bit of chocolate syrup, this works wonder for me, as it satisfies my sweet cravings and helps me to stay away from donuts, pastries etc.
To get more protein eat yogurt, paneer (cottage cheese) with your meals. It ok to eat 3 meals a day ( eating smaller meals a day is purely a myth and its not superior to eating lesser meals )
Include lot of green vegetables ( in the form of salads mainly ), as micronutrients in them aids in the chemical processes in the body which helps build muscles.