how to install drawer slides but are scraping against drawer?
1/16" is going to make a difference here. Very small margins. 

Don't rely on those metal flaps as shims. If you need shims, use shims. 

You have not done anything wrong, you've just entered the PITA adjustment phase. Higher-quality hardware has adjustments which can be made after the hardware is installed. These slides are not that. You are fortunate that your problem is the width and not vertical because you'd hate to have to move the glide 1/16" up or down - the screw would keep getting sucked into the old hole. 

Take it apart and flatten out those "shims." Then put it back together and try it. Based on the math, it won't be quite right. Add 1/16" shims to both sides and try that. If you have measured correctly this far, you'll be done. Obviously if it doesn't work, try 1/16" shim on only one side. Get some serious shim material that you can measure and which won't compress. And whatever you do, don't strip the holes with all of the in and out. 

Whatever that other answer said about support in the front does not apply. These glides don't need support. If they require support it's not done properly.