Open a new window with any size of your liking. I chose 300x300

Paste a picture that you want.

Make another layer with the same picture, position it the same as the original. Click on the top layer > colors > curves. Position the curves to what i did, at least the background is whiter than her hair and body. Then click ok.

After that, click colors > desaturate > and pick one of your liking. then click ok.

After that, click invert which is also found in the Colors tab.

Color in character white and the background black. Make sure you color the background and character properly.

It will turn out like this.

After that, right click on the original layer, Add Layer Mask > White > Add.

A new layer will appear and it will look like this. I know it'll say something like, "Cannot create new layer from the floating selection because...." so click on the anchor icon beside the dustbin icon (circled in red). After that, design your poster the way you want it to~ I hope this helped <3

RESULT: