There is a total of 6 items that once belonged to Johnny Silverhand that you must collect in order to unlock this achievement.
1 item is missable (Johnny's Pants) see below.* After completing the main story mission “Tapeworm” which is at the start of Act 3 of the story, you will get the “Chippin’ In” side job (no matter what dialogue choice you use).
Johnny's Tank Top - Automatically obtained after completing the main story mission “Tapeworm”. It will be in your inventory (Upper Body/Inner Torso).
Johnny’s Aviators - Automatically obtained in the side job “Chippin’ In”. It will be in your inventory (Head/Face).
Johnny’s Jacket (Replica of Johnny's Samurai Jacket) - Automatically obtained in the side job “Chippin’ In”. It will be automatically equipped.
Johnny’s Gun (Malorian Arms 3516) - Obtained by picking it up from the floor beside Grayson (“Take gun” option in dialogue) in the side job “Chippin’ In”. It will be automatically equipped.
Johnny’s Car (Porsche 911 II Turbo) *NOT REQUIRED FOR THE ACHIEVEMENT* - Obtained in the side job “Chippin’ In” by getting the access card from Grayson by either not drawing your weapon on him (“Put weapon away” option in dialogue) then following the rest of the objectives in the side job or by killing Grayson then looting the access card off of him and finding the car yourself by lowering the crane. Johnny's car is not required for unlocking the achievement.
Johnny’s Pants *MISSABLE* - Obtained during the Gig “Psychofan”. After getting off the elevator and going inside apartment 1702, it can be found in a suitcase in the bedroom upstairs. The Gig “Psychofan” is located in southeastern The Glen, Heywood.
This Gig should appear for you early on in the game.
This item is missable because you will not be able to re-enter the elevator that takes you to apartment 1702 after you complete the Gig.
Location:
Johnny’s Shoes - Obtained during the Gig: “Family Heirloom”. It is looted from the same locker the bootleg recording/bootleg shard (which is the objective) is in. The Gig “Family Heirloom” is located in southeastern Charter Hill, Westbrook.
You must reach Street Cred level 37 for this Gig to appear.
Location:
This is a video showing the locations of all 7 items: (All credit for this video to HarryNinetyFour)