Let
\documentclass{report}
\usepackage{sagetex}
\begin{document}
\sage{2^1234}
\end{document}
be named by example.tex
0) Run pdflatex example.tex for creating a sage-file
example.sagetex.sage
1) Find out where is your bash-executive bash.exe from your Sage-installation, it is something like
C:\Programme\SageMath8.9\runtime\bin\bash
2) Find out where is your "sage-starting-file" sage [without extension] (which is getting startet by bash.exe, see later), it is something like
C:/Programme/SageMath8.9/runtime/opt/sagemath-8.9/sage
3) Realize which is your current working folder, say
C:\Users\ccc\Desktop\Test\SageTeX
4a) In the following sage-run you need to change Sage's home directory to your current working folder from No. 3; the sage-command is
os.chdir('C:\Users\ccc\Desktop\Test\SageTeX')
BTW: You can check your Sage-Home in a Sage-Shell by typing in os.getcwd()
4b) And you have to run your sage-file; the sage-command is
load('example.sagetex.sage')
Alltogether in the Windows-cmd:
C:\Programme\SageMath8.9\runtime\bin\bash
-l C:/Programme/SageMath8.9/runtime/opt/sagemath-8.9/sage
-c "os.chdir('C:\Users\ccc\Desktop\Test\SageTeX');
load('example.sagetex.sage')"
Use this without linebreaks!
5) If all works, Sage will say to you: Run
pdflatex example.tex
again. After that you will get your sageteXed example.pdf.