Asking how to make Welsh Rarebit (which I assume is the purpose of your post) is a bit like asking how to make a curry! Which version do you want? There are hundreds of them! For me, any decent Welsh Rarebit needs to contain beer (but not all recipes include it).
Any 'spread' which says, on the packaging, that it's suitable for cooking should do. (I've not tried it with Welsh Rarebit but I've often substituted olive oil for butter in recipes, with good results!).