Let's look at the last part: "... mn tht qb a..." — "mn" could be "in" or "on". If m→i: m(13) to i(9) = -4; n(14) to n(14) if word "in"? Then "tht" would decode with same -4: t(20)-4=16→p, h(8)-4=4→d, t→p → "pdp" — not "that".
Given this is a puzzle, and you asked for a "write-up", I'd conclude the intended decoding is a (or 21 forward), yielding: Download- fydyw tjss ly lhm mharm mn tht qb a...
I suspect the actual answer is a simple ROT13: ROT13 of "fydyw tjss ly lhm mharm mn tht qb a" = f→s, y→l, d→q, y→l, w→j → sqlqj — no, that's gibberish. Let's look at the last part: "
Actually, try common English phrase: maybe "fydyw" = "could" or "would" or "every". and you asked for a "write-up"