So, using the system I've described (just look for the label "double" or "DSIP"), there are only a few ways you can penalize the opponents. Perhaps the most common is when an ostensibly takeout-oriented double is passed for penalties. This usually has the advantage that the extra strength and the extra trumps are in different hands. The other situations arise when double occurs (1) after one of us jumps, thus making a relatively clear statement about the distribution; or (2) after a redouble; or (3) after we make a cue bid, and (4) subsequent to several circumstances having to do with having told your story sufficiently often or having previously shown that you didn't want to compete further.
And then there's the easiest of all contracts to double for penalties: a notrump bid which is "to play." My definition of to play is when it's natural and (1) at the two-level or higher and/or (2) when it's in competition.
Here's a nice example of a low-level double of a competitive notrump bid which was easy to apply and quite profitable (9 imps). It's from an IMP table on BBO (I have redacted the names, although I don't think anyone really did much wrong*):
Those N/S pairs who played in notrump made either 8 or 9 tricks (best defense holds it to 8). But all other contracts were destined to go down. So the important thing here was to maximize the penalty. There was nothing particularly difficult in finding the red card but I will make a couple of observations. First, one of the most important factors in a penalty double is a lack of fit with partner. The worse the fit, the fewer total tricks there are likely to be and the more we want to defend a doubled contract if we can. Second, my hand had something in all the other suits so it would not be easy for them to wriggle out of it. And third, I trusted my partner to have a real overcall. It wasn't what I would call a great overcall, especially vulnerable, but it had solid opening values. Partner was not particularly interested in a spade lead, and he wasn't taking away any bids from the opponents, so therefore he was competing for the contract. And, given his outside strength, there was even a decent chance that, if I had a spade fit, we might even be able to make game.
Of sixteen tables that played this board, there were two others at which the first round of bidding was identical. But in one case North passed, in the other he bid 2♣. A few Wests chose to make a negative double instead of the 1NT call. To my mind this is equally dangerous because North has an easy redouble. But that never happened in practice. Instead, those Norths chose to bid notrump, which predictably did reasonably well.
The moral of the story is simply this: While it's nice to be able to diagnose a likely penalty, both partners must be on the same page regarding the meaning of the double in order for the trap to be successfully sprung.
* There may be some arguments that both East and West slightly overbid their hands, especially considering the vulnerability. But these were not particularly unusual actions. 11/16 Easts opened one heart. 3/10 Wests hearing one heart and one spade then bid 1NT. Really, I think they were just a little unlucky.