It is easy to make a spice powder from roasted coriander seeds. All you need is some coriander seeds and a spice grinder. The medium-sized jar that came with your mixer grinder will do just fine. This video shows how to do it.
Two things to take care of while grinding any spice: One, do not burn the seeds, and two, grind only after the seeds have cooled down.
Toasting the seeds before grinding is important, but we end up burning the seeds most of the time. That happens because the toasting process is allowed to last longer than necessary. The flame should be turned off immediately when the seeds start to release their aroma.
Then, transfer the seeds to a plate and wait for them to cool down fully. Once cooled, the seeds should be ground but again, keep checking after every 5 seconds. See, the already toasted seeds will heat up again during the grinding process, and if that is allowed to last longer than necessary, the spice will burn.
As you will see in this video, the grinding needs to stop when the powder is still coarse.
The advantage of grinding coriander seeds at home is that you can use a fresh powder whenever you need it. Actually, grinding a few seeds using a mortar-pestle set is also not a bad idea. Use a freshly ground coriander powder once, and you will know how good it is compared to a packaged powder.
Video credit: Renita Pais YouTube channel.