Ok, so i start by laying my curb strap on the ground, so that when it's finished the buckly will be on the top side, like below
then put the first metal thing by the buckle like so.....
then put the curb through the bit
and then through the metal under the other leather part
then put on the next metal
then feed the leather through the bit
and then through the leather on top of the other one
and now it should work, enless you twisted one end, which happens a lot in snaffles (if you are using a snaffle) but just redo it...
it should look like this
Hope this helps!!