It already has a modchip?
To start, download and extract the latest hekate.zip onto your SD card. READ THE README on the front page to learn your options. Configure it.
Then place the hekate....bin on the root of your SD card, and rename it payload.bin
Then Grab the Latest Atmosphere and extract all of it to your SD card. READ EVERYTHING IN
https://github.com/Atmosphere-NX/Atmosphere/tree/master/docs and
https://github.com/Atmosphere-NX/Atmosphere/tree/master/config_templates, and configure that too.
You will also want to grab Lockpick_RCM and place it in /bootloader/payloads, and Launch that payload from nyx and get your keys.
And that's just the beginning. You'll likely want to search here for "how to create your own sigpatches".
And about a billion other things. But this should at least get you started. Do NOT install files onto your EMMC. Learn how to make an EMUemmc first, and do all of your installing there. Immediately backup your emuEMMC as soon as it's created, so you can use it to restore your emmc, in case of softbrick. No matter how many times you make backups after that, don't ever lose or overwrite that backup. It'll be the cleanest backup you can expect to have with a modchip installed.