You question is too general. A template for ...what ?
Quote: How do I create a template?
First thing is you need is a size. Gimp images are composed of pixels (look it up) and a template has a size of xxx pixels by yyy pixels. Do not use centimetres or inches unless you use one of the Gimp standard templates, top of the File -> New dialogue where the pixel sizes are set for you.
If it is for social media then you need to know the appropriate size, do a search and you might find something like this: (note the use of pixels)
Facebook Image Size – 1200 x 628 pixels
Twitter Image Size – 900 x 450 pixels
Instagram Size – 1080 x 1080 pixels
LinkedIn Size – 1350 x 440 pixels
Pinterest Size – Pin size: 236 pixels x Adjustable Height; 2:3 ratio for vertical pins (recommended) Board image size: 222 x 150 pixels
Snapchat Size – 1080 x 1920 pixels
Tumblr Size – 500 x 750 pixels
Go to File -> New and use the required pixel sizes
https://docs.gimp.org/2.10/en/gimp-file-new.html
To help in positioning your work, frequently a template uses guides see:
https://docs.gimp.org/en/gimp-concepts-i...uides.html These only exist inside Gimp and not there when you finally export as a png or jpg.
....and that is your basic template, you might want to add color or a pattern.
Go to File -> Save, give it a name and save as a Gimp .xcf file. That keeps all layers, guides, selections ...
...and that is your template. When you work on it Save with a different file name to prevent loss.
Quote: How can I select from my pics and cut and paste into a template
All sorts of ways, some better than others.
Make a selection using one of the selection tools:
https://docs.gimp.org/en/gimp-tools-selection.html
Copy the selection. Edit -> Copy
Go to your template and paste it in Edit -> Paste As -> New Layer.
If the wrong size / wrong place correct using the Unified Transform Tool.
https://docs.gimp.org/2.10/en/gimp-tool-...sform.html
I have deliberatly use the Gimp documentation as references, All of what you require is there.
A couple of animations (quick - do not blink) - might / might not help.
New Image as a template: New / use FB sizes / add guides / Save As
https://i.imgur.com/Rbuvq0C.mp4
Open / Select / Copy / Paste:
https://i.imgur.com/I40wTQh.mp4