How can I start drawing in a Window called by DoModal()

[Sedai]

I have a main dialog window. By pressing a button, I open a nother window with DoModal();
Then I use

CDC *pDC;    
pDC = GetDC();
CPen blackpen;
blackpen.CreatePen(PS_SOLID, 1, RGB(0,0,0));
pDC->SelectObject(&blackpen);
pDC->MoveTo(100,100);
pDC->LineTo(250, 350);

However, the above starts drawing in the main dialog window. How can I make it draw in the newly opened dialog.
Thanks!
Avendeval