Totally there nothing wrong with aforementioned toilet ward but it has been pointed out in the books that it is easier to anchor wards to thresholds than it is just to throw them up. Of course in some books there are examples of people and things just throwing up wards.
Thresholds are much trickier to establish.
In my opinion, a toilet bowl ward would be a real pain in the...
gluteous maximus.
As for Thresholds, there isn't anything special done to establish them. Thresholds are borders between things. Night into Day is an example of a threshold. The fence or wall around a cemetary is another example of a threshold. Other examples of Thresholds would be the external walls of buildings like churchs and homes, or the internal walls separating different apartments in a building. All of these are examples of Thresholds.
Now, as a result of the Homestead laws (from Storm Front I think...) the metaphysical Threshold around a home (as opposed to an office building) or a church or other place of worship can be quite strong. Things which tend to make the Threshold stronger are individual structures (a house vs. an apartment in an apartment building), a female presence instead of just being the home of a bachelor, a family living in the home, etc. Basically all of the things which make and tend to reinforce a place as being a home, instead of just being somewhere to eat or sleep.
The way they are established and strengthened is by having the building built, and then used over time, preferably by the same people and/or family. As such, there is no real way for a wizard to 'build' a Threshold. Either a place they're at has one (i.e. something other than a Threshold of Mediorce +0) or it doesn't, the only way for that to change is for the Wizard(s) to add Bless This House to their respective lists of Powers, or establish a family in the place where they want a strong Threshold, all the while doing the normal sorts of things a family does.
Mechanically, a Threshold would have a certain value, depending on many of the factors I mentioned aboved or as mentioned in the books (novels and RPG), on top of which a skilled Practioner would also like have a Ward of some kind. Using the little stone cottage of Glenda, the Good Witch of the North as an example, there would be something like as follows:
Threshold: Fair (+2), Glenda lives in the cottage by herself, but has lived there for the last twenty years. Plus Glenda also has Bless this House (automatic +1 to Threshold strength).
Ward: Legendary (+8) Glenda has put some effort into Warding her home, enough so that if someone were unfortunate enough to crash into it, the cottage would likely escape major damage, but the car would not.
Now, if there was a hostile caster of some sort, attempting to cast spells at or into the cottage, in order to breach the Ward around the cottage, either to damage the Ward of someone/something inside, they would need to be able to direct a Legendary (+11) shift spell, just to get 1 shift of effect past both the Threshold and the Ward.