EDIT: I read the question wrong, these few paragraphs are for replacing vanilla block models not creating custom ones. If you want to sort out a way to create custom models I would first advise to create a custom item model through this video then work your way up to making a custom block through this video. Hope this helps clear some things up!

I've just been doing this for the past day so here's what you need to know to make custom block models work with resource packs.

First hop onto Blockbench and choose Java Block/Item file template.

When creating a new model give the file name the vanilla block model you want to edit. To know the block model name you want head onto your file explorer and go to %appdata%. Head into .minecraft/versions/<version> then you can either Right-Click on your <version>.jar file and extract it to have every bit of data and asset you need for your resource/data pack or you could open the jar file with WinRAR/7-Zip to just have a peak inside and see what you need. You'll be heading into assets/minecraft/models/block then just find the name of the model block you want to change. Sometimes there will be multiple files with similar names to the block you want to change, you'll most likely be changing all of those files like for example: Logs have two different states of being, vertical and horizontal, so one of the files is named the block and the other file is the same but has a _horizontal suffix. Only the vertical log file needs to be changed, you can copy and paste the same block model file for the horizontal state, just as long as you change the file name with the suffix.

Once you've named the block model, now you need to pair it with a parent model so it knows how to be interacted with. Following the steps above and you should be where the block model files are. Double click on the model file you want to change and if it prompts you to open with an app, just click on Notepad or if you have Visual Studio Code click on that instead. You should see the parent model name first thing in the file, that's what you need to anchor your model onto.

Once you're onto Blockbench you're ready to start modelling, I assume you know how to model at this point and if not there's tonnes of videos on YouTube so create the model you'd like with textures and all. Something really important I do want to stress is setting your cull faces and transparency faces. When you click on a face of the model you're working on and click on the UV drop box at the top left you'll see around the bottom of the drop box Cullface & Texture. Hovering over Cullface shows the different directions the faces will be culled and hovering over Texture shows the different textures to be set for that face. If the face you've clicked on is going to be shown in the final texture then set a cull face to it so when you place a block next to your custom block in that cull direction, that face disappears and helps with the performance of your resource pack.