Most hardware stores and home centers sell premixed compound in sizes from a quart to 5 gallons. A small container will be enough as it has a lifespan. It will be good if opened for a year and if you add a little water it will last longer. They also sell blades to put it on with from a couple inches wide to 18”. There are two types of tape made for Drywall a paper product that needs to be stuck down to a thin layer of the compound and then covered with compound and a mesh tape that has sticky on it and then when you cover it acts as reinforcement against future cracks. That is the one I like for jobs like this. Get the compound on as smooth as you can, but if you can’t it can be sanded very easily with sand paper or special sanding cloth made for drywall compound. Let it dry a day sand and then give another thin coat, repeat. If you use the tape it requires a little build out so the tape is covered. Most of the time when you feather it out over a wider area you can’t tell. YW