This is the process that works for me, but your mileage may vary. Customize it to work for you.
(click to expand)
Start your day by clicking Start Checking, then open Slack, p2s, your phone, etc. When you've gone through the important stuff, close Slack or put it into do-not-disturb mode, put your phone on silent mode, and place it upside down or in a drawer.
Click Start Deep Work. Start working on a project, and focus on it until you reach a milestone, get tired, are ready for something new, etc.
When you're tempted to self-interrupt and check Slack/etc, resist that, and refocus on the project. If you feel anxious about missing something, resist that. It's fine, because you're just AFK for a defined block of time. Once that block is done, you can check stuff again. Everything will still be there. If something is truly an emergency, your team can push through DnD mode.
Take breaks periodically, ideally before you get tired [link to how it takes longer to recover]. There's no need to stop the timer. Pick a task-negative thing to do-- like using a foam roller on your back, going for a walk, making a cup of tea, washing the dishes, laying down and closing your eyes, stretching, etc -- so that your mind can process the things you've just been working on. If you do something task-positive-- like watching a YouTube video, playing a game on your phone, etc -- then you'll be putting new stuff into your brain; you won't get the insights from processing, you'll make yourself more tired rather than less, and you'll have to pay the costs of context-switching when you start working again. When you're feeling rested, don't check anything, just sit back down and pick up where you left off on the project.
After you've made meaningful progress, feel ready for something new, or it's been 3+ hours, write down a brain dump of where you are in the project, and what the next steps are. Then, click the Start Checking button, open Slack/etc, and go through the important stuff. If you're checking for a long period of time, you can take (task-negative) breaks during it too.
Repeat the whole process until you're done with your day.