How to get onto DSP asap?

I've suffered with atopic dermatitis for 8 years now and for the past year it's become extreme to the point of constant daily pain and flare ups. Some days I can't even move and lately it's caused my mental health to rapidly decline.

I have an appointment with my dermatologist soon, but what documentation or evidence would I need to gather for a DSP claim? A statement from my dermatologist and GP? I have no idea where to start but I can't keep going like this, my medication and costs of maintaining my condition is taking a huge chunk out of my pay at this point. 😞