The level of detail required to map the distribution of an invasive plant species on your site or in an area will depend on the size of the site/area and the purpose of your management plan in addition to the number of different invasive species present. Good record keeping is essential on both large and small sites.
In these situations, it may be sufficient to do a line drawing of your site and mark the distribution of the species on that drawing. You should take fixed point photographs to show the extent of the species. It may also be useful to mark on your drawing areas where you are certain the plant is not present. Doing this will allow you to refer back and assess the progress of your management plan.
For larger sites, it is recommended that you use a GPS device and GIS system to map the distribution of the species. You should also make some effort to estimate the density of the plant/s. This will help you to gauge the amount of effort and/or herbicide it will take to successfully bring the plant under control. It will also allow you to go back and measure the success, or otherwise, of your control programme.