Choose “Select and Patch a File” in the method, and select the stock boot/recovery image.In most cases, it should already be automatically checked. If you are patching a recovery image, make sure “Recovery Mode” is checked in options.Press the Install button on the Magisk card.Copy the boot/recovery image to your device.If you are still having trouble, go to XDA-Developers and look for resources, guides, discussions, or ask for help in your device’s forum. You should be able to extract the file you need from official firmware packages or your custom ROM zip (if using one). If your device does NOT have a boot ramdisk, you need a copy of the recovery.img If your device has a boot ramdisk, you need a copy of the boot.img

(P.S.2 If you are interested in how Android boots and how it affects Magisk, check out this document) Patching Images (P.S.1 If your device has boot ramdisk, you can also install it with Custom Recovery) If you are using a Huawei device and the SAR result is Yes, please check its own section. If you are using a Samsung device and the SAR result is Yes, please check its own section.

Magisk zip installer