Looking to develop a payment plugin

I can see over 30 payment plugins available for HikaShop, but I
don't see one for Sage Payment Solutions. There is one for
SagePay, but evidently that is not the same thing.

I found this in the developer documentation:
How to create a HikaShop plugin

Are there any instructions specifically for creating payment plugins?

What would be the suggested steps for creating a new payment plugin?

Thanks.

Depending on your experience level, the simplest thing to do is look for a payment plugin that has a similar payment processing method to what you are trying to connect with. Then download that and look at how it is laid out to use as a guide for your custom plugin.

Also, see if Sage Payment Solutions can emulate Authorize.net. That may be a quicker solution (but beware, some payment processors charge extra to use this feature, which can add up over time).

You can always get a development company to build a new credit card processing payment plugin for about $300 - 400 USD, depending on the exact features you need. Of course, depending on your programming skills which influence how long it would take you to develop and test it yourself, that price may or may or may not be attractive to you.