Powders are what they sound like - stuff ground up into a fine dust. Powders are just a specific form herbs - different forms have different uses. Sprinkling powder around the house or dusting a candle is simply more practical than filling a candle with intact leaves or throwing entire Devil's Shoestring branches around the home.
A love powder is therefore ground up herbs that are magickally inclined toward generating and/or drawing loving energies and forces. This site has an articles section which includes herbs (http://www.spellsofmagic.com/featured_articles.html) - just take a look and find some suitable herbs.
7 is a good Venusian number, and Venus is the planet most associated with love. Seven herbs would therefore be a good number to work with.
It will help you by drawing loving energies into your life and strengthening those that are already there. The SPECIFICS of how it will help depend on how exactly you use it. For example, you can...
1 - bake it into ritual cakes if the herbs are safe to eat (and presumably don't taste terrible);
2 - dust a candle you intend to use in a love-magick working, thus helping empower the candle with loving energy;
3 - sprinkle it around your home or room to generate a more loving atmosphere;
4 - combine it with a flammable binding agent to create love incense;
5 - sew it into a pillow or sachet that will encourage loving energies in you via aromatherapy;