Tying knots is all about technique. Each person has to work out what is best for them. I have found that you have to use both hands together to tie knots. Even when I thread a tippet through the eye of the hook I use both hands together. It is not just holding the knot with both hands but using both hands to hold the knot as you progress with tying it. I can't post pictures but I have some already posted that may help.
The following doesn't help you tie an Improved Clinch Knot but I wanted to illustrate that you need to work your two hands together. Here are some photos that shows how I hold the tippets when fastening two loops together. Notice how I use both hands to support each other.
This picture shows the first step placing one loop over the other. Nothing fancy here.
This picture shows how I grip the two loops with my off hand to hold every thing in place while I do the next step. I am in complete control and ready to do the next step. This griping the knot as you proceed is a key in tying knots and will help you with your Improved Clinch Knot.
My left hand is holding the knot together and with my free hand I pick up the end of the line and insert it into the loop.
Now you can see it all coming together. The left hand has the two loops locked together. You see I have the tag end of the line I want to fish through the loop held between two lower fingers. Then I am picking it up with the fore finger and thumb of my right hand and pulling it through the loop.
I hope you can see I have developed a method to hold the parts of the knot as I progress with the tying. Everything is under control. You can do this same thing with your Improved Clinch Knot. Hold things in place with your left hand and do the tying with the right.
I hope this makes sense to you.