It's like this.
If its a tough matchup you build like this, swap into this build whenever you realise
relic shield, refillable potion, also ask sup or w/e to go rs as well
beserkers grieves - movespeed and aa, king item
red ward - vision, replace when dies
static shiv - farm back into the game, core item
gaurdian angel - stopwatch is infinite scaling defence item
if you haven't died - dont die, work with team
if you have died a lot and worth nothing - keep running it down a lane shoving cs and getting gold and respawning with a lower level death timer, maybe skip ga and just leave it at bf sword #1 and go into the following, finish later when rejoining team
after this go into lategame with a reaver/ie/bt (first bf item ga doesnt count) and pd is the classic combo
this setup is most gold effecient if ur struggling in a hard matchup at any level, this is how to play adc from behind and still be useful.
if its not tough, tier 2 boots (either serk swiftness or mobility, hurricane & rageblade)
if its a contended game more likely ninja tabi and dorans blade then attack speed, sometimes bf rush