Recommend me techniques for making instructional videos

I agree with GuyUK​ - you need to start with a script. Once you have your plan for your video content mapped out, you have a wide variety of tools to work with: from simple, free and easy to use Adobe Spark (Free Online Video Maker | Adobe Spark ) to very comprehensive and powerful Premiere Pro.

You might benefit from building up your own skills library first, and you can do this along with other educators for free on Adobe Education Exchange: Adobe Education Exchange  - take a look at their upcoming course list, including Explanimations, Multimedia Presentations, Digital Storytelling, and Documentaries.

Also, you may have access (for FREE) to Lynda.com/LinkedIn Learning training through your school system or local public library, and they have an extensive collection of videos --> that's my daily go-to! My favorite is the Getting Started series from Ashley Kennedy:

Welcome from Premiere Pro CC 2018 Essential Training:  The Basics by Ashley Kennedy

I go along with Premieregal and would underline what she says about using a tripod especially if using an iPhone.  Get an adaptor if necessary but avoid wobblyvision at all costs

If you want to record your own voiceover, I'd recommend getting a base level USB mic, such as this RODE NT-USB, to record directly into your computer to Premiere Pro CC using the Audio Track Mixer.

If you need to create any screen recordings, I'd recommend Capto for Mac and BandiCam for PC.

If you need to record your keyboard as you showed in one of the sample videos you sent, you can record with an iPhone or another video camcorder or DSLR using a tripod and then you can edit the video to watch the screen recording and your voiceover.

While I agree with Jim, if really want to do it yourself, the most important thing is to get your ideas down on paper.  Put them down in a chronological order, with an introduction explaining exactly what the the point of the video is, then progress the video describing each step until you reach a logical conclusion.

Write the complete piece out using double text spacing on the left hand side of each page.  On the right hand hand side roughly sketch or describe what you want to see and hear to correspond with the text on the left. 

Once you have it in this format, you can fine tune various points and you will effectively have a shooting script.  When you have done this, you may realise that you need to take Jim's advice and a pro will be able more easily be able to work from your script.

If you do opt to produce it yourself, discuss your script with the participants and potential viewers and then adjust if necessary.  Doing all this on paper, will point out the method of shooting and hopefully the pitfalls. 

Above all keep it simple.   

Good luck.

I recommend hiring a trained professional, just as you're students have done.