How to book The Last Supper in Milan
Welcome to this forum, clengh4. You will find that this board's Search function (top center, blue, on desktop) works rather well. Then you don't even have to wait for replies:
Go here to make your reservation. I think they are booking through April right now. It's fairly simple to make your reservation. The times we have went before (last time was March 2019) we had to exchange our voucher for a ticket at the onsite ticket office which is just up the street from the church. We are planning to go again in June.
Good luck!
Here is the link to the site for purchasing the tickets to The Last Supper Tour in Milan. I thoroughly enjoyed my visit there. Some posters have commented elsewhere that they had to notify their credit card company in order to complete the purchase. Follow the link in the previous post to read how they handled that issue. God luck.
Hopefully you can book through the sights provided. However, it was my experience a few years ago that everything seemed to be sold out. Tours buy them up quickly. If you cannot find tickets available on those sights, don't give up, try booking a private tour that includes the Last Supper. I used Viator tours and found a very reasonable priced tour which included the Last Supper.
https://www.viator.com/Milan-attractions/Da-Vincis-Last-Supper-Il-Cenacolo/d512-a727
or ask your hotel's front desk for help