For me, one of the easiest ways to build a gallery wall is to start by clearing off the current wall. I find it best to start with a clean, preferably white, canvas. All the walls in my house, for the exception of my family stairway gallery wall, is white. That wall was supposed to be "Destiny grey" from Sherwin Williams, cute little BFF pun for my bestie, Desty. From all the sunlight exposure on the large 20 ft ceiling window, it turned lavender! Cute hue for that area but I will keep my white everywhere else, kthxbye.
I gather frames from everywhere: Dollar Tree, Goodwill, IKEA, Home Goods... You name it, I have bought frames there. This helps me keep the variety going. I also love wooden wall decor with motivational quotes or inspirational sayings, even bible verses!
I use pictures from trips that I have taken, free prints online, my own printables from my shop, or tear-outs of magazines. Using old boxes, hats and trinkets, I fill in the empty spaces that are left on the wall. My latest favorite that I have added into my gallery wall piece collection [I recycle all my decor onto different walls when I get into a "I must change everything in all my spaces" mood] is my Hanging Cork Trivets with Scrabble Tile Letters DIY Project. I made it last summer with a lot of love and very few materials. Easiest. Tutorial. Ever.
My grandma's Liz Clairborne beach hat, burlap wish baggie, box of Habana classico tobacco cigars and others are the ultimate gallery wall finishers. Use what YOU have to make YOUR home unique, lovely and fully of all things YOU! Enjoy your spaces and make them yours.
Hope this was helpful in some way! Feel free to ask me any questions in the comments below or the live chat box on the right hand corner of your screen for a faster response.