I'm about to adjust the intonation of my strat and I was wondering if it was best to do with new or old strings? I've read from different sources that older strings are better because the strings are already stretched out, therefore better intonation, is this true?

New strings. Always on new strings. Even when the string stretches it wont affect intonation unless you have a bad string.
You can do it on old strings but as soon as you put new strings on you have to do it again. Better just to check em every new set.

Also dont waste your time setting intonation by led or needle tuner. You will get better accuracy doing it by ear 12th fret harmonic to 12th fret note.

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Thanks for the great info Niki, also how often should I check the intonation?

I'd check it every string change. If you just run down the harmonic to fretted note method, quickly everytime you change strings. Every 6 months do a full setup and spend more time going through the intonation method i mentioned.
Stick to that and you wont go far wrong. Also i set my guitars up when the seasons change rather than a strict 6month timespan. Start of winter and mid spring, its roughly 6months.

When you use the harmonic to fretted note method for the first time be incredibly picky. Really spend ages about it, one string at a time. Your ears will tell you far better than any tuner.

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This is so true. the only tuner I ever used that seemed to be as accurate as even my tin ear was a really really expensive strobe tuner. Like over $300.00

So I decided it is easier to take my time, and trust my ear. Also i hate the idea of relying on electronic tuners anyway. playing guitar is a hearing art, tuning your guitar by ear seems a valuable skill, pitch recognition will help your playing. Don't get me wrong, i still occasionally reference the A 440 , but not all the time, and I find I am super close to pitch when I do anyway.

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I'm not an expert, but I play one on the internet.

If you use the same strings it may not change between string changes--the main factor will be if you travel with your guitar through different climates--or if your home experiences wide climate changes.

Those could (but may not) affect the wood in the guitar requiring an adjustment of the truss rod or the intonation.

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It wasn't Willy-Nilly, it was at crows.

The only tuner I have is an old (expensive) strobe tuner.
I believe it works great for setting intonation.

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My other guitar is a Strat.

How can I do the method of intonating the guitar without any electronic device, I'm a newbie and don't want to screw up

Its easy.

Play the low E string harmonic above the 12th fret. Compare that to the 12tgh fret note. If the note is flat compared to the harmonic, move the saddle for that string towards the nut. If the fretted note is sharp in comaprison to the harmonic. Move the strings saddle back towards the back of the bridge plate.

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