it is always recommended to wait till after 6mths as their system is more mature n is less likely to have allergy reactions to solid. if your baby is putting weight well n your pediatrician never ask you to start so early i guess better wait for another 2mths? anyway she gonna be on solid for the rest of her life. no hurry?
if you really wanna start, start with 1 teaspoon only 1 time a day then slowly increase the number of teaspoon day by day but stick to 1 meal a day till about 1 month later then increase to 2 meals a day. for how much to give baby, you can refer to my girl's blog here Our Precious Princess!: Nutritional Needs of a Baby (per meal)
for what to give and what to take note (eg the 4 day wait rule), read this site
Wholesome Homemade Baby Food Recipes, Make your baby healthy homemade baby food with our easy baby food recipes, solid food tips, baby nutrition & more!
4 day wait rule=give only 1 kind of food for 4 days at least before introducing another new food. this is to see if the baby has allergy to that certain food or not. also to see how the little body digest the food.