How should "vegetarian duck" (seitan) be pre-prepared to substitute into a duck recipe
It's often a good idea to drain mock duck and bake it in the oven to slight crispiness (10-15 min at 180°C). You can add some cooking oil of your choice (something refined and smell-free is better) to make it fatter if you don't mind extra fat. If your brand of duck is not spiced, you can also try to add some white pepper/five spice mix or something you like there. If liquid in the tin does not smell good, you can soak chunks of duck in some stock of your choice prior to baking.

There is almost no way to make it fool a meat-eater that it's a real thing except in some very spicy dishes where you can't really make out any individual ingredients. That said, in Thai noodle soups and curries or fried with vegetables or in gravy it is less recognizable than being fried without additives.

So try different recipes and see what feels better. I certainly recommend you to try Thai and Chinese recipes, those guys are expert in mock meats. Not that there're not some very nice Vietnamese, Cambodian, or Malaysian dishes.

For example, I find mock duck the most suitable for Guay Teow/Kwai Teow (has different variations in Chinese and every SE Asian cuisine, you can google lots of them, or try some variation of it in every single vegetarian restaurant in Thailand and is super easy to make with just a few special ingredients and ready stock), at least much better than mock chicken (which is better in Tom Kha or curries). Hopefully, you will find something suitable.

Note that after baking mock duck doesn't really require much cooking, add it one of the last when frying, or even directly to the serving bowl for noodle soup.