It's really simple, and can be done in a very scientific manner.
- Get a packet of your "favorite" baja (or any other flavor) seasoning packet.
- Flip it over.
- Take the packet to the isle with jars of seasoning.
- On the packet, under "ingredients", buy every spice listed there. If you can't find it (like corn syrup solids), skip it. Also if you already own it, you probably dont need to buy it... but you get the idea.
- Add the ingredients into a bowl. The one listed first on the label gets the most, the one listed last gets the least, in order. If you really like one (like garlic), add more. Under every condition, add less salt. I usually add none to my mixes, because I can salt my steak/ground beef/gravy/fish/chicken/salad dressing as normal. Yes, I really do make all those things from scratch.
In the end, you will find that pre-packed seasonings are 3-4x more salt than you thought, and you will make something based off your preferences, not off of what an accountant said should go into a mix.