How much time should I reference in a chart?

Meaning… if I’m looking for a double bottom or top, what size time sample should I be looking back? Last 24 hours? last 15 minutes?

For example… GBP/USD obviously shows an uptrend in the 1 day view since April… but that’s not true in the 15 min. view.

What timeframe do you recommend I focus on to start out with developing a strategy?

That really depends on your style of trading, but say you think you have a major uptrend on the 5 min chart, but then you go to the 1 hour chart and see the whole chart is in a big downtrend, then you could potentially get duped so it depends on your style so if you see a double bottom and you are a scraper and work within 5 minute charts then go with that, but if you are not looking to get in and out then focus on checking different timeframes and you might notice a different trend to what you fought was happening. I personally have 3 charts with 3 different trend lines so 15 minute 1 hour and depending on what I am looking at the end chart is 4 hour or 1 day chart so I can see what the general uptrend or downtrend is and then I can focus more on what candles, indicators, support and resistance, news is saying then go from there.

The higher TF are better. A head and shgoulders pattern would be stronger than say one on the 5 min pattern.

That all depends on you. What works well for one trader can be a disaster for another trader. You need to find out what works best for you which is done by testing your approach in a live account with a small amount of money. Take a few hundred trades and then check the results, keep a journal so you can know what went on and then keep at it until you get the results you are looking for.