Equal parts clay and Plantex CSM+B, add distilled water until it's good and workable. Roll into little balls and put on a piece of non-stick aluminum foil or wax paper, etc. Let dry for a couple weeks.
You can add magnesium sulfate, KNO3, or just about anything else you want to the clay/plantex csm+b mix and it'll come out fine.
I usually do 4 Tablespoons clay, 4 TBSP Plantex CSM+B, 1 TBSP Epsom salt, 1 TSP Potassium sulfate.
You just push them deep into the substrate around where you have some plants, as you would with any substrate fertilizer tablets.
If not using a nutrient rich substrate, you could space out the DIY root tabs on the bottom of the aquarium, ( with your dusting of peat and mulm, etc.) and then put the substrate on top of them to give the plants a good start, then place in new ones as necessary.
I don't really know. I never put phosphates in the fert tab. I put KN03 in it only for specific tanks as well.. not every tank. I prefer to just regularly dose my macros and take care of them that way, and the plants don't seem to miss it much in the substrate. I look to use the DIY fert tablets more like Seachem Flourish Tabs, which provide mostly just micro nutrients.
Plantex CSM+B is a trace nutrient mix (dry), with boron. A person could do the same thing with any other similar dry micro nutrient mix, such as Miller Microplex, etc.... basically any aquatic-use dry micro-nutrient powder. The equal parts micro nutrient powder and clay powder should work universally. The important part is really the amount of water you use to reconstitute the clay and fert mix. You need to add water in a very small amount and very gradually, as it is easy to add too much. I use a pipette... squirt a bit of water in, work it in, repeat. Too dry and the balls will just crumble after you dry them. Too wet and you just have a mess.