Since it has and will no doubt continue to be occasionally popular to speak in "ye old manner", I thought I'd make a little post demonstrating how to do it properly. Now, if you really want to be a proper crusader, you should be using the Latin vulgate, but if you're an unschooled peasant or a filthy heathen, I'll cover the most important English rules here:
THEE, THOU, THY, and THINE
It's time to play "Who Remembers High School English?" Because, surprise mofuggas, that one day your boring teacher that smelled like mothballs was droning on about objects and subjects of a sentence and you swore you'd never care to know has come back to haunt you with the vengeance of a lonely old lady grading the essay you never spellchecked.
THOU – You use "thou" as the subject of a sentence (by which I mean if "thou" is the one doing something, not the one being done something unto). For instance: "You shouldn't have walked so close to the edge" becomes "Thou shouldst not have walked so close to the edge".
THEE – You use "thee" when "thee" is the object of a sentence (by which I mean the person being done something unto). For example: "I have conquered you" becomes "I have conquered thee".
THY – "Thy" is the possessive form, like "your" in modern English. However, it is used only in the same places as "my" would be used (as opposed to mine), and when the following word begins with a consonant (as opposed to a vowel). For instance, "Get your spear out of my head" becomes "Get thy spear out of my head" or "These are your mistakes" becomes "These are thy mistakes".
THINE – "Thine" is also second person possessive, but used where "mine" would be used, and when the following word would begin with a vowel (as opposed to a consonant). For instance: "Meet your end" would become "Meet thine end", and "These mistakes are yours" becomes "These mistakes are thine".
Putting it all together, we get something like: "Thou shouldst not have walked so close to the edge, for now I have conquered thee; thou hast fallen unto thy doom, and thine end is written in blood upon the earth."
Notice all the "shouldst" and "hast" and wonder what the rules are for those? Much simpler. The extra "t"s and "st"s are specific to second-person verbs. I have, Thou hast, He has. I should, Thou shouldst, He should. The "t' will end any verb that isn't an awkward consonant (for instance, you can't really pronounce "dt" and the like), and the "st" (or sometimes "est") will end any verb that you can't end with "t". Generally. As with most languages, there are exceptions, but this is a good rule of thumb.