Accept you are unwell. Act like you have flu, to take pressure off feeling like you 'ought' to do things. Cancel appointments and work if needed, because you are genuinely very unwell by this stage of depression, whether triggered by an event or just snuck up for no apparent reason.
I am always amazed by the difference between a duvet day ridden with guilt that I should be doing things and a day in bed which acknowledges I am currently too ill to do anything.
After a good 24-48 hours of bed rest I then tell myself 'You don't have to want to, you just have to do it' and go through the motions of self care: shower or bath including hair wash. There is something restorative to the depressed brain about washing your hair - maybe it's the scalp massage.
Then a small amount of healthy food. If depression triggers sugar or carb binges, still add this rule: something healthy first - boiled egg and wholemeal soldiers or fruit and yoghurt or a simple salad. If you can, add food known to trigger dopamine and seratonin boosts: most nuts and seeds, especially walnuts and brazil nuts; blue cheese, grapefruit, turkey, berries.
Then fresh air. If you feel too frozen for a walk, just sit in a sunny spot and focus on nature - clouds in the sky or leaves, birds singing, cats on walkabout etc. But if you can walk, do. No pressure to feel better afterwards, just breathe, look for signs of nature, walk as fast or a slow as you like.
Also - gentle stretches. Depression does cause physical freeze, so even a 2 minute stretch of arms, legs and spine can help, as can the weird but effective static run, where you put one foot forward, bend legs and lean slightly forward as if about to set off on a run, then stay still but just pump your arms. It really gets the blood moving.
Music might help. Very soothing piano music helps me, or songs that are upbeat but not too attached to memories of other times.
I find journalling helpful too - download the brain blocking loops of depressive thought and then gently challenge them. It's especially useful to list what you did. e.g. If your mind is ranting: You are so useless, you have so many things you HAVE to do but you have done NOTHING for days, it can be therapeutic to say, 'I am very ill right now, but even though I feel so ill, I managed to put on a washload, have a shower, call the GP, feed the cat.' And if you truly haven't done anything, it helps to say, 'Yes, I stayed in bed all day to rest because I am so very ill. Clearly, if I need another day or more in bed and am so ill I can't even wash or dress, I will know that I need help and make an appointment to see the GP.'
Just ditch all guilt, self-criticism etc. To get well, we have to admit we are ill and give ourselves recovery time.