This is about as detailed as I will explain it here, definitely not enough information to replicate the process. One key thing to remember is that a vacuum is used during the process, this loweres boiling points and allows the preservation of the THCa. You need solvent/s to get the cannabinoids in a solution during the processing.
You start with a basic extract, say BHO is to be used.
This extract is then mixed with hexane and acetic acid. These are two chemicals that, together, will help further separate the plant material form the THC. This happens by dissolving the plant matter, but keeping the good stuff untouched. This solution is then placed through a filter (buchner funnel, erlenmeyer flask, watts 1 micron discs and a vacuum pump) so that more plant matter can be removed. The remaining liquid will be placed in a rotary evaporator.
This machine will use different values of pressure, heat, and motion to further separate elements in the solution. After this process, the resulting liquid is called Δ9 tetrahydrocannabinolic acid. But this is still far from what we’re looking for. There will still be impure elements, so chromatography will help remove these. Sephadex-LH20, dichloromethane, and chloroform are added to our tetrahydrocannabinolic acid.
To further purify it, the chemists will place the crystals back into the rotary evaporator. These are then mixed with methanol to separate even more impurities. This solution then goes through a filter to remove these impurities and back into the rotary evaporator to remove the methanol. To end the process, this will once again be mixed with a dissolvent to purify the mixture to the 99.997% it’s famous for. This is finally placed in the evaporator once again to remove the harmful solvent. And there you have it; this is what creates THCA crystalline.