Blighty covers the subject, really all you need to know.
More specifically:
As Blighty said you will be scaling up by some amount, so expect some degradation in the image.
When it comes to using the scale dialogue, set the ppi to 300 before setting the new image size. Otherwise you tend to go around in circles.
edit: A revamp of the video showing the Image -> Print Size dialog to check and exporting as a pdf in Scribus since that seems popular these days.
In Scribus, there will be borders and bleed areas, I will let you sort that out. As a note, you could let Scribus do the scaling for you.
In the demo, Exporting from Scribus as a pdf. Gimp by default will always open any pdf at 100 ppi. It will be 300 ppi but in Gimp up-to-you to set that value in the open-pdf dialogue.
Gimp -> Scribus -> Gimp 4 minutes.
sorry video out-of-date and gone
More specifically:
As Blighty said you will be scaling up by some amount, so expect some degradation in the image.
When it comes to using the scale dialogue, set the ppi to 300 before setting the new image size. Otherwise you tend to go around in circles.
edit: A revamp of the video showing the Image -> Print Size dialog to check and exporting as a pdf in Scribus since that seems popular these days.
In Scribus, there will be borders and bleed areas, I will let you sort that out. As a note, you could let Scribus do the scaling for you.
In the demo, Exporting from Scribus as a pdf. Gimp by default will always open any pdf at 100 ppi. It will be 300 ppi but in Gimp up-to-you to set that value in the open-pdf dialogue.
Gimp -> Scribus -> Gimp 4 minutes.
sorry video out-of-date and gone