Help on Images / Mockup Designs in Photoshop


New Member
Hi, I wish I would've stumbled on this website before now. I need some help on creating and saving mockup images of T shirts in Adobe Photoshop for use on new ecommerce website. I am placing a PNG piece or art on a TIFF model/mockup image.

Please answer questions, tell me if I'm correct or not, or what I need to do so it's done correctly! This is what I've recently been told..
1) Image should be 72PPI? (Right now it's at 300)
2) Where and when do I make that size adjustment? Do I make this size adjustment under Image > Image Size? Do I do it after I've placed the PNG art onto the Tiff Model?
3) When Saving, I click FILE > EXPORT > SAVE FOR WEB
4) Save as JPEG High Quality 60. When saving, do I save FORMAT as IMAGES AND HTML or just IMAGES?

Or do I even need to bother with this? Ive had some saying do it like that and others saying don't bother as it compresses it automatically..