Make a photocopy of the picture you want to use as the main image in the poster. For the best results, start with a black-and-white photo and blow it up to 8 1/2 by 11 inches. Use a photo of your band, either on stage, unposed or shot with a fish-eye lens. Or, use a photo of any random person or thing that appeals to you.
Stencil text on the photocopy in black or white paint, depending on how much dark or white space you have to work with. Alternatively, paint the text in freehand. For smaller text, use a sharp-tipped permanent marker. If it's a poster for a show, be sure to include the name of the venue, street address, time, cost and whether the show is all-ages or 21 and over.
Add more details in paint or marker, if you like. Don't make it too fancy, and don't make it too dirty. Allow the paint to dry.
Photocopy the poster in the quantity needed. Most old underground grunge posters were only 8 1/2 by 11 inches, so no special printing process is needed. Alternatively, scan the poster and have it printed in any size you prefer.