There are 6 items to collect:
- Johnny's aviators
- Replica of Johnny's Samurai jacket
- Johnny's tank top
- Malorian Arms 3516
- Johnny's pants - Obtained during "Gig: Psychofan" (missable)
- Johnny's shoes - Obtained during "Gig: Family Heirloom" (missable)
Johnny Silverhand's car can be obtained during "Chippin' In" but it's not required for the achievement
To unlock Johnny's pants you need to do the "Gig: Psychofan", it's a side quest given by Sebastian "Padre" Ibarra , you will need to do the first 4 gigs for 3 others to appear, one of those new 3 is "Gig: Psychofan". You can get the pants once you reach the objective "Find Kerry Eurodyne's guitar", before taking that guitar check the room next to the one where the guitar is and you will see a suitcase with the pants in it. If you're not stuck somewhere I recommend checking a YouTube video about the gig. Just remember that you can't go back to take the pants again if you miss them during the gig.
To unlock Johnny's shoes you need to do the "Gig: Family Heirloom", it's a side quest given by Rogue Amendiares, to get this gig, you must complete Wakako Okada's gigs and you will need to be Street Creed 35 or more. Once you're doing the gig the shoes are kinda hard to miss since you got to loot a stash to continue the quest and the shoes are in that same stash, you can also check a YouTube video about it.
YOU CAN MANUALLY SAVE THE GAME BEFORE STARTING THE 2 GIGS IN CASE YOU NEED THE PANTS OR THE SHOES SO YOU CAN RELOAD THE SAVE LATER.