First, unfold to one chorus as BB likes to call it.

Then it's a simple matter of copy the part and paste it to make the song longer.

Since you understand how to slow it down by adjusting the measures.... simply do that.

Don't forget to set the tempo back to the original speed after the slow down parts.

If you want to make the tempo change smoother, go a step further and split the measures into measures of 2 beats as opposed to 4 beats. Then you can slow each one progressively more, thereby making the tempo change smoother to the ear. I did this on a tune recently and I believe you must add new measures..... so if the tempo change is over 3 measures..... you would add 3 measure and change all 6 of them... 3 original ones and 3 added ones..... to TWO beat measures.... the project will not sound any different because the number of beats remains constant..... but now, you can change the tempo down in speed 6 times with smaller jumps as opposed to 3 times with larger jumps.

hope this helps

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.