I believe nothing in design should be rushed, but especially the process creating a gallery wall. Making your own gallery wall will and should, take a long time. First there is the process of choosing what wall to use and how large you want the display to be. This usually isn't too difficult, you may choose a large empty wall, a space above a TV, sofa or a bed, or even in a stairwell. You should measure out the space and decide how much of the wall space you actually want covered. For a gallery wall above a sofa, bed, console table.. ect., you will probably want to create an imaginary line between 3"-12" above the furniture top, where the gallery will start and then work your way up no closer then 3" to the ceiling, or crown molding. This is a good tip for any spot as you really don't want to get too close to the ceiling (I made that mistake once) and you want to keep the center of the gallery around eye level. An easy way to anchor a gallery wall is to use ledges and have frames lined up and overlapping each other all on the ledge, this will make it much easier to change up or add to your gallery wall.
Next there is the process of gathering frames, art, mirrors and any other wall art that inspires you and that together will create a cohesive look. This may end up being the longest step as it takes a while to collect many pieces. You want to choose pieces that are not only inspiring, but also that go with the colors and style of your space. Personally choosing art is one of my worst skills and being on a budget just makes it twice as difficult. Since I chose such a large wall, I knew I needed multiple oversized frames so my latest purchase was 4 floating gallery frames in black, sized 20x24" from Pottery Barn. I love these frames because they fill up a lot of space and they are considered 'floating' because you can put any sized print inside them. For now I choose 4 prints from a John Derian picture book that I tore out and framed, using prints from a book is a cheap way to get art and a good way to ensure it will look cohesive. I have already changed up my gallery wall numerous times, and I'm sure I will be changing it again before too long, so that's why I like having frames that the prints/photos can be switched out easily. For a more eclectic look you would want to do a mix of old and new pieces. Maybe begin by finding pieces you already have and asking family members if they are looking to get rid of anything. Bonus points if anyone in your family was an artist, I have a painting that was done by my great-grandfather (top left corner of my finished gallery wall) and Brian has one done by his grandmother (hung opposite of our gallery wall) and they are our favorite pieces not just because they are beautiful but because they have meaning to both of us. Any art that has some type of meaning to you is going to be more inspiring and will always hold a story. Also bonus if you are a traveler, try to find artwork around the world that inspires you or frame photographs from your trip, this will give you a very interesting mix and will be a nice memorable piece from wherever you found it. Any gallery wall that has more of a collected look will have a lot of interest to it. Not saying this is the only way to go, a wall of all the same color frames (in different sizes at least) can still make a big impact in a space when done right. Choose an interesting location, and have art or photos that are inspiring and meaningful.
Once you have collected enough wall art for your gallery wall, it is time to decide on a layout! Some people might simply layout their art on the floor and then hang it, but I like to see everything visually and took my time on this step. I first created a layout to scale on paper and cut out the sizes of my frames so I could move them around and rearrange on paper. I thought this was very helpful in choosing a layout and choosing how high and how spread out my frames should be. Most gallery walls work best when frames are spaced about 3-4" apart. After I had finally decided on a layout I then taped out the location of the frames onto the wall. This helped me visualize the display on a large scale and I perfected the placement of the layout. Once I had the wall taped up I had decided to move the whole display about 4" to the left, as it was too close to the doorway on the right. Because I did my gallery wall on such a large wall this step was necessary for me, and my wall isn't exactly centered in the room so I wasn't quite sure where to center the gallery wall until I had seen it taped up.
And finally the last step is hanging your wall art! This is also something you do not want to rush as if things are uneven it will offset the whole design. Whether or not you decided to tape up the whole wall, using some painters tape is very helpful when finding the placement for the nails. Simply flip the frame over and use the tape to 'measure' between where the nails need to be and then place that tape on the wall, making sure it is level, then place a nail on each end of the tape, or one in the center for smaller frames just mark how far down the nail needs to be, and simply peel the tape right off. Hang your frames and repeat until your gallery is complete, then sit back and enjoy your hard work!