Cardboard is an option that some mounters used to get the 'job' done quick and easy. However, if you want to make a proper job of it, you will need to use Buckrum. As I mount medals myself, I use it for a more professional touch even though it will not be seen after the medals are mounted.
Having had to take apart quite a few sets of medals to remount and add new medals can I please, please implore you NOT to use glue. It leaves the ribbons unusable and can also 'bleed' through the ribbon and can be seen on the front of the mounting. I had to remount a NATO medal as the guy couldn't understand why the white on the ribbon had turned yellow. Turned out it was due to a big blog of clear glue that had aged badly.
It will take you longer but I always sew the medals on. Make sure you chose a decent backing aswell. Some of the remounts I've had to do were all 'top show'. A pretty good job from the front but a right mess on the rear, making it look a bit haphazard and unproffesional with bits of glue showing and thread hanging off. Some may argue it doesn't matter as no-one will see it but if it's my medals being mounted, I'd prefer someone to do a proper job of it.
As previously mentioned by Fallschirmjager, check out the british medal forum. There are some well clued up people on there who will be willing to point you in the right direction.
Good luck