please tell me how to use jigsaw blocks!

The basic idea of jigsaw blocks is: You put them at the edges of your structure pieces. Each one has a "name". After placing the first structure, the game will find other structures that also have jigsaw blocks with the same name. Then, it will align the second structure to the first so that the two jigsaw blocks are facing each other. Then, the two jigsaw blocks are removed.

All that said, I don't think MCreator lets you create actual structures, only features, and I don't know if jigsaw blocks work on features. Even if they do, I find that more interesting structures can be generated using procedures.

I would actually recommend not using jigsaw blocks. They're useful, but also very limited in what they can accomplish. Whenever I do procedural structures, I use custom blocks to generate them. You can make a modded block that does something similar to a jigsaw block, (disappearing after updated, for example), but then runs your custom code. (And can thus be configured to your heart's content.) 

For a simple example, I made some procedural dungeons to replace the vanilla ones. They're still a simple one-room design with four side rooms, but instead of just having one structure, the structure itself is made of a single, custom block. When the structure loads in, a procedure manually updates the block. On update tick, the block first makes sure its surrounding area meets my generation requirements, then randomly chooses a dungeon-center-room structure, and places it in the world. The central room has four additional custom blocks on each side, each of which randomly selects a side room, and rotates it based on the block directly behind them. Then, a third custom block in the middle randomly destroys blocks, spreads, moss, cobwebs, etc; making each dungeon unique. You can extend these principles to far, far more complicated structures.

A couple things to keep in mind: Blocks will not tick naturally when placed in the world from structures; so you'll need to either run a procedure on structure generation that updates all your custom blocks; or make sure that the thing placing the initial structure is itself a single block that gets updated on generation. Also keep in mind that this system means that structures will be generating after world generation, meaning you'll also have to take trees and stuff into account. 

I am trying to make a new item called a blueprint, it works like a structure block but you need the materials to make that structure. When it is right clicked, it opens a GUI with input slots and a button. When the button is pressed, it generates a simple house I made in 2 minutes. How can I do this without making a procedure that places each individual block with the [Place block {} at xyz] block?

https://www.youtube.com/watch?v=B5rwJZeeXWs&feature=youtu.be

There is a new tutorial on this that can be found at https://youtu.be/6_ZelkGxAlQ?si=H48i65aOX3K_o3-e

I highly recommend checking it out :)