1. Design the layout of my poster on a piece of A4 paper. This is usually a rather quick step (which often makes me consider hand drawing my posters, but that is another story). However it is essential to ensure a smooth process afterwards, and also to make me realize when there is no way the amount of material I want to share is going to fit.
2. Prepare the content. I create a LaTeX file with the equations I want to insert (and checks that everything renders as I want it when compiling), a text file with some of the largest chunks of text I will want to insert (actually, it's often mostly my references, as I try to write down as few words as possible), and all my figures. By the way I usually make graphs with matplotlib (as I usually code in Python), diagrams with Dia, and everything else (including drawings of mice) with Inkscape.
3. In Scribus:
• Create a document with the right size (e.g. A0) and orientation (landscape or portrait).
• Lay out frames (text frames, image frames and render frames, which will contain the LaTeX equations). There's a bug in the version of Scribus I use which makes the frames disappear even with the option to make them visible checked (in the View menu), therefore I set their border color to something like Magenta to stop losing them while I'm positioning them. Having the grid visible helps hugely.
• Define colors (which you only need to do once), under Edit > Colors.
• Create styles (which you only need to do once) for the title, authors, section titles, frame text, and so on. Use the "Properties" box (available with F2) to assign styles to frames.
• Insert content into the frames.
• Tweak the results until I'm happy with the results. Highlight words, rearrange frames, adjust margins, remove the magenta borders, and so on.
• Finally, export to PDF. For better rendering of the LaTeX equations, check "Embed PDF & EPS files" in the "General" tab of "Export > Save as PDF". Under the "Color" tab, select "Printer" in the "Output intended for" menu. Check the results carefully. Put on USB key. Go print.

Regarding LaTeX rendering:

• Insert LaTeX equations using render frames. Right click on the frame, pick "Edit source...", enter your LaTeX code in the left-hand box, and the preamble code (\usepackage, \newcommand, and all things that would come before \begin{document}) under "Fonts/Header" in the right-hand box.
• There's a small problem with the original LaTeX editor configuration file, which limits the width of equations to that of an A4 sheet of paper (with margins). To fix this, edit the file 100_latex.xml (or create a new one, and pick it under Preferences > External Tools > Render Frames). For me this file is under /user/share/scribus/editorconfig.
Replace
\documentclass[a4paper,$scribus_fontsize$]{extarticle}
with
\documentclass[$scribus_fontsize$]{extarticle}
and
\usepackage[left=0cm,top=0cm,right=0cm,bottom=0cm,nohead,nofoot, paperwidth=$scribus_realwidth$pt,paperheight=$scribus_realheight$ pt]{geometry}