Making a template for fabricating a pipe boot from sheet metal
You have to make the round vertical piece with the angle cut on it's end first. Get that cut nicely so that it meets your needs even if it takes a couple of iterations to get the correct angle and the shape when it sits on a flat work surface. 

The next step is to position the angle cut part on the flat sheet in the center area. Use a felt tip permanent marker to trace around the outside of the tube piece onto the flat base piece. Cut out the oval shaped hole along the inside edge of the marker trace line. After cutting the residual marker can be removed with rubbing alcohol.

If you decide to have bend up tabs to rivet and solder to the vertical piece you will have to arrange for those before you set out to cut out the oval hole.