Your statement "playing with AC" is a bit worrying. I am sure you know that mains voltage AC can kill you very easily and horribly painfully too, so, be very very careful. Home made fuse boxes for mains AC are really a no no. Do yourself a favour and get the real thing as there is a very good chance your 'piece of wire' will not blow fast enough for some things you connect to your power supply.
Regarding winding transformers, I'm sure the exact info is to be found on the net if you do a search. What I did was to take an existing transformer apart, leaving the primary winding undisturbed. If you had measured the secondary voltage and the maximum current you could get out of it you have all the info to rewind the secondary for a different voltage if you count the No. of turns while taking it apart.
Keep in mind that you CANNOT increase the power output from a given core size, you can increase the voltage with a consequent smaller current or vice versa.
Building transformers from scratch is not easy, first, where will you get the laminations from? Then you have to do all the calculations, make a bobbin, find the right winding wire and layer insulations etc. etc., much easier to rewind the secondary of an existing transformer if you find one that comes apart easily (not all do).
A word of warning about variacs: usually they are auto transformers, which means the secondary has a direct path to the mains - a lethal device in the hands of a novice. Take care!
I suggest you do not use it directly on the mains but use a double wound transformer with its output fed to the variac instead. It does not matter if the double wound transformer steps the secondary voltage down low, the variac can step it up again, this time 'isolated' from the mains. You can still get zapped by the output voltage but you are no longer getting zapped directly from the AC mains. Which means, if you are handling some earthed equipment while playing with the AC from the isolated variac, you will not get an instant shock if you touched just one wire, only if you touch BOTH wires from the variac.
Klaus