What I used:
Double sided tape
Your photos (I got free prints from Photobox)
Not included in this photo is the sponge I used. Rather than buying a pack of brushed I just used the foam part of a kitchen scouring pad. Much cheaper!
The bit that took the longest was deciding on the layout of the photos and then measuring everything out. It was difficult to be able to include a mix of landscape and portrait photos but there were 2 portrait ones I really wanted to include so I made them the centres. I would suggest just choosing whichever photos you want and then having a rough play around with the layout.
Next, after measuring everything out and making small pencil marks for the photos, it is time to stick them down. I put a small piece of tape on each corner of the photo and then laid it onto where I wanted it. What I wish I had done is put tape along the entire edge of each of my photos! The problem I had after is that some of them lifted up off the board between the bits of tape.
Once all your photos are stuck down it is time to cover them with mod podge (instructions are on the bottle). This was the scary bit because I felt like I was going to ruin the photos! But it does dry clear after several hours so don't panic. My biggest tip would be to make sure that it is all spread flat because I have a few blobs of it over some of the photos.
And this is the finished product! I am still yet to put it up on my wall at uni because I need to buy some sticky pads that are none damaging (not sure the landlady would be too happy otherwise). But it makes such a great decoration and a way to display lots of photos at once. Would love to know if you ever have a go so leave links below!