If you Google "grids", you will find an abundance of opinions as to which works best. Many people use sacred geometry to create them, others use various templates you can easily find online and print off. Personally, I prefer to use my creativity and allow my intuition to guide me.
Creating crystal grids is one of the most empowering way to use crystals. The best advice I can offer is to not allow the process to overwhelm you. There are no "rules" and you can't do it "wrong". The more grids you create, the easier it becomes and before you know it you're using this tool on a regular basis.
Let's talk about protection. It is important to set energetic boundaries in order to keep ourselves from harm. Intentional or not, negative energy tends to cling to us and spreads easily. The best way to counter it is to be proactive.
Tourmaline is an extremely protective stone, dispersing negativity and banishes fear. It protects against ill-wishing, disperses stress, and clears negative thoughts while encouraging a positive attitude.
Smoky Quartz is also very protective and brings positive energy to a space. It also relieves fear, replacing it with a sense of calm and teaches you to release what no longer serves so that you can move forward.
Selenite brings immense peace and tranquility to a space, while preventing negative energy from intruding. Placing a piece of selenite on either side of a doorway has been known to literally stop negativity in it's tracks. It is also extremely protective, especially of children.
Once you have chosen a spot and taken the time to create your grid, it's important (in my experience) to clearly state your intentions. Ask that your space is protected and that anyone who has negative energy or intent will not be permitted to cross the threshold. I usually go one step further and ask that anyone who is up to no good will not be able to find the space. They will simply move on and hopefully abandon their plans in favor of something positive.
Pay attention to little nudges from the Universe. If you sense that your crystals need to be cleansed and charged, then feel free to do that. One way I recommend is by placing the crystals in the moonlight for three full days and nights, starting the day prior to a full moon and ending the morning of the fourth day. Then you can reset your grid and allow that protective energy to work it's magic.