To gain muscle mass as a vegetarian, you need to consume more calories than you burn. This means diet will be the most important factor in gaining weight. Because cardio burns the most calories, you will want to limit the amount of cardio you do. Instead of high-intensity cardio, such as running, you could do more low-intensity cardiovascular exercise, such as walking, using an elliptical machine, or slow-cycling. Focus on strength training, as it will be your most effective way of gaining weight as a vegetarian. You can get a gym membership to access weight machines, weight benches, and barbells/dumbbells, or you can set up a simpler at-home gym. Learn some strength-training exercises you can do with weights, including bicep curls, bench presses, and squat presses. You can also focus on bodyweight training, strength-training exercises that don’t require weights. Some examples are planks, push-ups, and lunges. When strength training, focus on either the upper or lower body in each session. You can train on back-to-back days as long as you aren’t working the same muscle groups two days in a row. Make sure to give yourself at least one day of rest per week so your muscles have time to heal.