I pulled up the carpet to install my 1/2 inch engineered wood flooring and I found the existing wood floor under the carpet. I was told I can install the engineered flooring in opposite direction of the existing floor but I had questions about it. What do I have to do to prepared the existing floor beside screwing down any loose or squeaky boards? I have a few boards that need to be replaced and I'm not sure how to replace 30 years old boards. Where do I get the replacements?

Pictures below:
https://drive.google.com/file/d/1MHbNOTUo0ii9PhA3Z0pNBShChI42m925/view?usp=sharing

https://drive.google.com/file/d/1fWf3BvaP6vBEi8Vy8MW5i68J3VQ6Q4K4/view?usp=sharing

https://drive.google.com/file/d/1WZ8nbWyt1H7Z7BnH9_TgYv2B3QJkOScZ/view?usp=sharing

Screws for squeaky areas will be fine. To find a new board you will need to find the width and thickness. A wood floor distributor will have the correct replacement. Since you will be laying floor over the existing, it will be your sub-floor so you would not need to use a hardwood. Fir, pine, or plywood close to the same thickness will be fine. The replacement should be within an eighth of an inch of the height of the old flooring. A cabinet shop could make a board for you.

Thank you. I also have a question about what blade I should buy for my miter saw and jigsaw for this kind of a job.

A fine tooth combination blaade will be good for power miter saw. A 6-10 TPI blade for the jig saw.

Thanks for the help on the blades. I am going to securing the subfloor and laying down the floor tomorrow.

I just noticed that I have two low spots. Is there something I can put down beside self-leveling cement to even out those spots? I think I saw a rubber mat you can put down in a youtube video some time ago. Anyone know what that is called?

How low? Thick felt can be used to flatten out minor issues.

1/8th and in one spot 3/16th of an inch. Can that be made up with felt paper?

It's tough to get perfection as far as flatness. If you use thick tarpaper you would need to use a marker to outline where the layers need to fill. I staple the paper down one layer at a time. With an eighth inch tolerance you can use eighth inch paper, which is the thickness of 90 pound roll roofing, or the single layer of three tab roofing shingles. The flooring will "bridge" a small gap just because it is pretty stiff and hard to bend.