Same way you isolate total RNA from cells normally. Either a kit or Trizol. I've had great success with Trizol reagent for isolating a lncRNA and detection/amplification by qPCR and regular PCR.
There is a paper where the authors talk exactly about it, if the TRIzol method can be used to extract miRNA from cells, in which cases it will work, and which no, besides that they suggests some strategies to increase the amount of miRNA you can extract by the TRIzol method from cells, it is very useful. I mean, I had this problem during my doctorate, because I did an in vitro model, and I needed to extract miRNA from those cells. And as I cultured cells in a small flask (5 mL), not in a petri dish, I didn't much total RNA.
Article Short Structured RNAs with Low GC Content Are Selectively Lo...