Try to plan ahead. This helps you know how many grids to buy and what size coroplast (or flooring) you needed. Have all your supplies ready to go so you can build it all at once. Duct tape is good for the coroplast and dowels help support levels. Some wire cutters are very helpful to cut the ends off the zip ties.
Some brands have larger holes, these are not safe for rabbits. Either only get smaller holes or use wire mesh to cover the grids. The larger hole ones can be used for levels (as they will be covered). You may need to open the box in store to see, but it can save you a lot of trouble later.
Use zip ties instead of the connectors. They are much easier to use. Even if you use the connectors, you would want to use zip ties anyway to help stabilize it. You would want at least 2 ties per side you are connecting (anywhere 2 grids meet), you can use more if you want as well. Some dollar stores have good quality ties, but not all do. 200 is a good number to start with for a good sized cage.
When you actually build it, start with small things. I usually do a 2X2 grid square and add on to that to get the size I want. If you are doing it taller than 2, then make it taller and add on. Levels are easier to do 1 grids at a time and connect them once they are attached to the sides. A few extra girds are good to measure out the bottom (a bit smaller if you put it inside, bigger if you put it outside the cage). I but chew guards which can be J plastic moulding from a hardware store. It is much easier to put them on when you make the cage than try doing it later. For the door, I like it to be 2X2 grids so there is easy access. A clip to keep it shut and some zip ties for the hinge. Access too each level is good for cleaning and to get your rabbit if needed.
