Since the top plate of your pier is not drilled, you can install your pier as soon as your footer is ready. If you are pouring concrete, you can set your anchor bolts into the wet concrete and the allow the concrete to cure around them. It's best to have a template made of plywood. You drill holes, matching those on the bottom plate of your pier, in the plywood. Then you install your anchor bolts in the template and use that assembly to keep your anchor bolts spaced and aligned properly while you sink them into the concrete and tamp the wet concrete around the bolts to ensure there are no air pockets around your anchors.
If you're building your footer out of blocks or bricks, you'll have to wait until after your footer is built and the mortar is cured. I'd still suggest a wood template. Use the template as a guide for a hammer drill (you can rent them at Home Depot) to drill holes in your masonry footer. Then, using anchoring epoxy (also available at Home Depot, but also Grainger, Fastenal, etc.), glue all-thread studs into the holes and let the epoxy cure.
Once that's done, you can bolt your pier to your footer.
I suggest looking up the solar transit time for your location for a day when you have time in the afternoon to work on your pier. At the time the Sun transits, you can use a straight edge held plumb to mark a line which will be aligned north-south. Now you have a reference for north. If you've already installed your pier, that reference line can be directly on your top plate.
If you don't want to make a plate for your mount yourself, contact Dan's Pier Plates, he may already have a template for a CEM-70 and be able to make you a top plate. If you want to make one yourself, you can get small aluminum plates from Grainger, or Metals Supermarket, or even eBay and Amazon. iOptron likely has a dimensioned drawing of their mounts' bases so you can make yet another template, this one from paper, to guide you in drilling holes in your top plate and the plate on your pier. Aluminum is very soft, so regular wood drill bits will do just fine. Just use some oil for cooling and lubrication, and don't use the drill's highest speed. Make sure you align your template with your north marker.
I wouldn't bother tapping the plate on your pier. Simply install your rat cage bolts and put a nut on each bolt below the pier plate and one above the pier plate. That will hold the bolts in place and you'll be ready to install another set of nuts for your pier plate (with you mount bolted to it) to rest on. A fourth set of nuts over the top of your pier plate will hold your mount down.
If you really want to tap your pier top plate, just buy the correct diameter tap and go slowly. Once you start cutting threads, I wouldn't turn the tap more than 1/4 to 1/3 of a turn before backing the tap out to clear the chips. That way you won't "tear" any of the threads.