This is a description of a process used to isolate alkaloids from acacia bark or phyllodes, but is based on a general process that can and is used to obtain alkaloids from other plants.
Obtaining the plant material is obviously the first step, in the case of acacia bark it is probably healthier to prune branches and de-bark these, rather than leave strip wounds on the stem or trunk of the trees. This leaves only a relatively small wound in comparison to the bark bark obtained. The phyllodes from the branches can also be used in this case. With many acacias pruning will actually assist the plant if done properly, as borers which live on and kill the branches are removed with the branches and may actually extend the life of the plant.
Plants in National Parks are protected and best left that way, roadsides and other disturbed areas are the first places to look, and generally enough plant material should be available without anyone having to take material from any National Parks. Some of these plants are available in nurseries, and people are working towards having all of the plants mentioned here available, so keep an eye out. It's recommended that people who utilise these plants give something back by planting and growing them, in few years self sufficiency from cultivated plants would be possible if you have a bit of space or know someone that does.

All materials such as ethanol, acetic acid or vinegar, ammonia and solvent should be free of detergents, perfumes, colours or other additives, these may affect the reaction or prove harmful. Electrical elements should be used for all heating as open flames are highly dangerous and likely to set everything alight. Excellent ventilation is also a necesity.

The plant material is dried, weighed (if possible, as this will in the end give you some idea of what yield you are getting) and ground as fine as possible, which can be a bit of work with the bark, which is strong and fibrous.

Then soaked overnight in 50/50 ethanol (and /or methanol, 'metholated spirits') to acetic acid solution (white vinegar), ph @4. Then heated until boiling and simmered for an hour or so, then the liquid filtered off and kept. Then re-cover the material with water, or more of the above solution if readily available, and repeat this step, without soaking overnight. It may be repeated a third time, but most of the alkaloids should be in the liquid that is taken off the material. The plant material may be composted/ discarded after this.

All the liquid is combined and reduced through simmering to a workable amount, about 500ml to 1l, depending on how much you started with.

To the concentrated liquid add about an equal volume of ammonia hydroxide solution (cloudy ammonia 3-5% NH4OH), which causes a pink precipitate to form (at least with the bark it does) at first, changing the solution from deep red-brown to a pinky-brown colour, ph usually about 8-9. Small amounts of Sodium Hydroxide are added to bring the ph to just over 10, at which point the cloudiness dissapates and the solution turns a very deep red-brown colour (always add small amounts of sodium hydroxide to the solution... definitely not the other way around, as this may cause an explosive situation, keep sodium hydroxide in a dry sealed container).

Then a solvent (at about 30-40% of solution volume) is added to the solution and shaken vigorously with it. The solvent should be a non water miscible liquid, generally substances such as chloroform, tetra-chloroethane and ether are used for this purpose as they have low boiling points. The chlorinated solvents can however be extremely toxic to come into contact with, and the non-chlorinated solvents are generally highly volatile and flammable so require extreme care. I have found that a cheap and easily obtainable solvent (in Australia) is what is generally sold as 'lighter fluid', it is less volatile than ether and not particularly dangerous to work with. Though other solvents would probably be better if obtainable without too much problem, it does work better when added to a still warm basic solution rather than 'cold', the solution usually still reatains heat from boiling down and then addition of caustic soda which creates more heat when added to the solution.
Upon standing the solvent should form a layer either above or below the aqueous solution, depending on the solvent, with an emulsion layer between the phases sometimes. Generally there is a marked colouring to the solvent layer, in this case a pale yellow-gold clour due to the alkaloids in solution, but the colouring doesn't necesarily give a real indication of how much alkaloid is in the solvent.

The solvent layer is seperated from the aqueous phase with a seperation funnel or pipette. This is then shaken with a small amount (a heaped teaspoon of each to about 100ml) of clean sodium hydroxide and salt (NaCl) solution. This is intended to clean out some, if any, impurities in the solvent other than alkaloids. It is then seperated again and if available shaken with an anhydrous salt to remove any water that may have been absorbed by the solvent, these steps aren't necesary, but rather serve to clean up the final product. A second and even third wash of the aqeous solution with solvent is recommended to obtain all the alkaloids, repeating these last two steps and keeping the solvent from each.

The combined solvent can then be left out in trays to evaporate, hopefully leaving crystals of alkaloids as it dissipates. Warm dry conditions are best for this, and differences in humidity and heat do seem to have a large effect on the final product. Once all sign of the solvent is gone the material remaining can be scraped up and is in fact the alkaloids in free-base form. They are quite reactive in this state and need to be kept in airtight dark containers.

That's it! But it does take some practice, especially if you don't have much experience with plants. Due to the fact that it is a plant product you can expect variation between different plants, in terms of content and final product which can vary in it's colour and smell, it is not the same as producing 'synthetic' or pure chemical compounds.