The conventions.
The traditional parts of a letter are really placement decisions, and each one is easy to get right on purpose. Here is the whole page at a glance.
The Date
Top right, a little below the top margin. It anchors the page and gives the letter its place in time; letters are kept, and dates are why.
The Salutation
On its own line, flush left, with room below it. Resist the urge to crowd it against the first paragraph; the pause is part of the greeting.
The Closing
Two or three words on their own line, placed right of center, with your signature beneath. The closing is a visual step down, not a full stop.
Avoiding the Awkward Ending
Plan the last third of the page before you write it. A closing squeezed into the bottom edge is the most common flaw on otherwise lovely letters. If space runs short, end a paragraph early; white space at the bottom is grace, not waste.