Warming up before beginning any penile exercises is essential because it increases blood flow to your penis. For Jelqing, you cannot even perform the exercise without being warmed up. However for length stretching they may seem less essential because it requires less blood-flow. Don't make the mistake of missing a warm-up routine, it can cause penile bruising and nerve damage.
If you find completing the warm up and warm down inconvenient then just perform your PE Exercises while taking a hot shower. The warm water has the same effect as a hot towel, and you can easily massage your penis to a semi-erect state while bathing.
I believe the best place to perform manual exercises is in the shower, because its so much more convenient.
How To Warm-Up Properly
Step 1: Put hot water on a small hand towel or cloth until its completely soaked. Squeeze out excess water, but leave enough so the warm temperature remains.
Step 2: Massage your penis to a semi erect state, around 50%. This is very easily performed in the shower with soap by mach masturbating, however it can be done outside the shower as well.
Step 3: Wrap the warm towel or cloth around your penis, completely covering your shaft and penis head.
Step 4: Perform 10-15 kegels while wrapped in the towel to get your blood flowing and your capillaries expanded and read for exercise.
Step 5: Remove towel or cloth and dry off your penis. Begin your penis exercise routine.
Your warm-up should take about 5 minutes, and after you complete your exercises you should always complete a warm down routine as well. It sounds time consuming but an extra 10 minutes to guarantee your penile health and virility is a small trade off.