If you want to do a gallery wall the first step would be to lay all your pictures out on the floor to get an idea of how they fit together best.
Source: DecoChic Interiors
Take note of the spaces between each picture - too much space and the collection feels disconnected, not enough space and it feels smooshed together. I find it's best to take a photo of the dry run on the floor once it's all done to use as a reference when hanging. It's easy to get carried away and forget how much space is needed or which picture should go where. If you plan to do a symmetrical design it is better to start with the centre picture(s) and work your way out on both sides. If your design is asymmetrical getting the bottom foundation row in place will make it easier to position the rest.
A trick to help you visualize how the gallery wall will look on your wall before you start hammering nails in is to make paper templates of all the pictures you plan to hang and tape them to the wall using painters tape. This allows you to see exactly how it will look and make any necessary adjustments.