A tip an old bench tech taught me that has saved me many times: I put clear “Scotch” tape over the case screws as they became “free”. The tape kept them in place while I lifted the lid off, cleaned it etc.
I purchased the kit from iFixit and everything has gone smoothly (so far - still on first charge!) on my mid-2013 11” Macbook Air. The new battery shows a manufacture date of Aug 2019 (3 months ago), was charged already to about 80% and shows “Full Charge Capacity (mAh) of 5045”. I concur with another comment that during re-install the connector has to be bent a bit to fit, but that was straight-forward – I did it before screwing the battery back in.
Overall this kit cost $30-50 more than something claiming to be the same from Amazon, but I didn’t want the hassle of a dud battery OR cheap tools that might might strip the many screws on a 6 year old laptop that has never been opened.
I concur with the Medium rating (mostly due to the tiny screws and how easily they could be stripped or lost). However, if you’re comfortable unscrewing electronics and take your time and don’t force anything, it will be hassle-free and Easy.
Many thanks iFixit team!