The transformer data will say the secondary is
0-18 (Let's call her Meg)
0-18 (Let's call him Harry)
So put one of the AC legs of the rectifier to the the 0 of Meg.
Put the other leg of the the rectifier to 18 of Harry.
Then join the 18 winding of Meg to the 0 winding of Harry.
You then get 36V RMS to the rectifier. This, surprisingly enough will give you 50V DC.
My concern is, and I don't want to offend you in any way, is that you are asking these elementary questions (fair enough) but are looking to explore the realms of wiring up to mains voltages.
We all have to learn, and you are learning a lot I'm sure (and so am I), but as far as mains voltages are concerned it does not take any prisoners. If you have any doubts at all, then you are best coming back to a PSU project when you have some more jolts under your belt. If you still want to pursue this, then it's probably a good idea to get some help locally off somebody that has carried out this kind of project and can guide you through this.
I imagine that the reason that more have not contributed to this topic is that you don't seem to have taken on the suggestions in the responses that had already been given (but it may be for an entirely different reason, I don't know).
Please don't be disheartened,
Trys