The post above is a good answer. He's my (probably oversimplified) slightly different (but mostly similar) version:
1. Gather a corpus.
(As with any statistical method, this is a sample, rather than a representation of the whole data set. So you want to gather the best sample possible-- large, contextually appropriate, well-transcribed, etc.)
2. Count the number of tokens of each word. Rank them. Make a list.
This is studied quite often and there is a Zipfian distribution to the words, so that the most common words are very common and the least common words are very uncommon.
https://en.wikipedia.org/wiki/Zipf%27s_lawIn other words, for measuring the MOST common words, you don't need a very large data set. (The most common word is "the".)
For measuring the LEAST common words, you need a much larger data set. Actually this is where it gets very difficult and where problems exist in corpus linguistics. It's not hard to measure common things accurately but very hard to measure infrequent things accurately.
A good test would also be to use several different sources. Does casual writing have the same results as conversational transcriptions? If the results match, you can be confident. If they do not, you can try to figure out why.
This website has a lot of information and is associated with corpora designed and maintained by Mark Davies, one of the leading researchers in this field:
http://www.wordfrequency.info/That's based mostly on written text, though. There are corpora of spoken English, if you want to do that.
One problem with your question is that you do not define "everyday life". It seems obvious, but you actually need to decide what this means. Spoken? Written? Native speakers only or everyone who speaks English? Where? Why? What topics? Frequency varies by discourse style, topic, participants, emotion, etc.