Everyone have slightly different techniques but this is what I would do.
I have cut out the area behind the tweet and mid for space, now I need to wrap the pillar in grille cloth.
Q: What is the best way to adhere the GC to the pillar and the rings?
A: You can use CA glue to attach the g/c to the back of the pillar trim piece. This will make for a better transition than gluing on the front only. If you are not planning to cover the entire trim piece at this time, you can CA glue the edges of the g/c that runs across the front. Personally, I would not bother with adhesive to attach the g/c to the rings since the f/g resin will take care of that for you.
Once done, apply enough resin to thoroughly coat cloth (Not over saturate) and let dry overnight.
Next step: Trim off excess grill cloth.
Q: What is the best way to do this and make it look "clean"
A: If you glue the g/c to the back there should be very little trimming. A razor blade will take care of any trimming you need to do.
I was them going to mix some short hair filler and resin and then pour in on the inside of the pods for reinforcement.
Q: Best way to do this? I got this from a few FG installers
A: Pour it in from the back. The trick here is to mix it thin enough that it flows but thick enough for it to stick and stay without sagging.
Cut out tweeter & mid openings. Sand grill cloth, gold body filler, sand, fill, sand until smooth and shape is what you want it.
Q: WHEN should I sand the filler?? when it is completely dry or somewhere in between?
A: For shaping, I like to use a cheese grater / perforated file. Stanley makes these under there Surform line. The half round is going to be your best choice. You shape when the filler has just firmed up but not hard yet. Never sand filler that has not fully cured because it will clog up the sandpaper and make it useless.
Once the pods are done, I want to prime it. The pillars have a texture to them, so I was thinking of using a high build flexible primer (SEM) to "fill it in" and prime it gray and smooth.
Q: Is there a better way to do this? they are going to be covered in grill cloth.
A: Since you plan to cover the pillar in g/c why bother filling in the texturing, you will not see it.
Once finished, I am going to cover them in GC as a finish.
Q: How does one adhere the GC to the pillar? and do you adhere it on both the front and back, or just the back?
A: If you are using g/c, don't glue the front, otherwise, the glue will show through the g/c. In general, any material that will absord glue should not be glued on the front. Glue it to the back. You can use spray adhesive or CA glue, I personally would use the spray. If you would like to soften the wrap, glue some 1/8" foam to the pillar and than wrap it. Just make sure to use a continuous piece and end the edges of the foam at the edge of the pillar and not short of it (it will look terrible). The foam can be glued down but do not glue the g/c to the foam.