I was planning on making that tutorial right now actually.
1. you need openBRF program, it's a must
2. open the Resource folder, brf file mmbrithats and find the glengarry_variant_scottish meshes that are towards the bottom.
If you want to say replace the british_rifle_shako_ranker all you have to do is rename british_rifle_shako_ranker to british_rifle_shako_ranker_old for example, and rename the Scottish shako/bonnet to british_rifle_shako_ranker or british_rifle_beret/_2 to replace a hat in-game as the Module System only reads items listed in troop inventories by mesh name in openBRF, I hope this helps and works out for everyone.