I don't know your exact situation, so I can only give generalized advice for you. We don't know if you have a 100 amp breaker panel, 200 amp, 400 amp, etc. We don't know what types of loads you are trying to power, whether your house has all gas appliances, or everything electric. We don't know if you want your 10Kw steam shower to operate with the generator or steam sauna, or olympic size swimming pool, etc (I've either run across all this or heard about it from other generator techs about absurd loads on undersized generators)
The above was just to say we don't know if the 803A will safely and easily power everything on your electrical panel, or if it would be massively overloaded and therefore you will have to shed some of the loads in order to safely run on generator power.
So as others have stated, your options are
doing an interlock kit (I have one for my 802A, installed when I had my panel replaced due to it talking to me). What kit depends on what brand/model of panel you have, so I won't give an example. There are many out on the market, usually in the $100 range for just the interlock. You will still need an appropriate size of breaker (50 amp for an 803A), wiring, and an inlet box rated to 50 amp. If you can do it yourself you're looking a couple hundred total.
a sub panel with just the loads you want to power (will likely require an electrician to install and move the desired loads over) Downside of this is if you change your mind later on what loads you want powered, you'll need the electrician back to run new wires for them. Panels start in the $200 range and go up from there; the electrician might charge you anything from $500 to several thousand depending on your panel location and everything else. Unless your panel is in the garage you'll likely want to use an inlet box again for the power cable rather than having to leave a door open for the cable. (bonus is it has load meters on it so you can see roughly how much power you are using)
example:
https://www.electricgeneratorsdirect.com/Reliance-Controls-TRC1005A/p1122.html
getting a manual transfer switch installed before the breaker panel, which will require permits (mine was a self filed permit that had to be posted on the panel, I don't think it cost me anything), your utility company removing your power meter and then reinstalling when finished, and an electrician to do the work of the install. You will still need to go into your panel and select what loads need turned off if you have too much power draw.
example:
https://www.electricgeneratorsdirect.com/Reliance-Controls-TWB2005DR/p1125.html
more examples:
https://www.electricgeneratorsdirect.com/power/manual-transfer-switches.html?sort_value=&displaynum=0&spec_options_id[1044][]=50
An automatic transfer switch will be useless for this unless you plan to get an autostart type kit in the generator designed for a transfer switch, as most require a 12V DC circuit to actuate the transfer mechanism (but not all) and will cost more than any of the above options. My guess is the panel will be in the $400-1k range, and install will once again be minimum $500 up to several thousand.
The majority of automatic transfer switches on the market are dumb switches, meaning they get controlled entirely by the generator.
You mentioned not wanting to just have it as a plug in and then turning off breakers yourself as this isn't the "right way" as you put it. (though I assume you meant using a suicide cord and just flipping off the main breaker, which is highly dangerous and illegal in every jurisdiction I have ever heard of, because linemen have really been killed by people doing this) Thank you for wanting to do it correctly, because as I mentioned, electricity can kill when people try to cheap out.
The manual interlock with power inlet is an approved and legal method of feeding generator power into a house. It's also the least expensive and simplest solution that doesn't require the generator to be permanently installed (not sure if it is a requirement where you are at, but many places if a generator is hardwired to a house it ends up requiring a lot more work/permits than just having it fed in through a cable you can unplug.)
Downside is unless you are near someone else on the same power grid, you have no way of knowing the power is back on unless you switch the power back to utility to check.
After that the manual transfer switch is the easiest/cheapest method of doing this. Requires additional work and contacting the utility company and permitting, but is the simplest solution.
Downside is the same as the interlock kit.
The sub panel route has more up front cost/complexity, but if you are worried about loads you need to shed before powering the house, then this is the best solution.
Nice thing is, depending on what circuits are transferred over or not, you usually have something plugged in or lights or whatnot that will stay on the utility feed and when they come on you know power has been restored.
Auto transfer switch, if set up to function, will only require you to start and shut off the generator. When power comes back it should return to utility automatically, so you just let the generator cool down for 5 minutes then go shut down.
