First of all, you don't need any special gear that isn't already lying around the room where you do your little home recording projects. You need a guitar, an amp, some cables, and a cheap passive DI (a really nice one will work in a pinch if you can't find a cheap one).
Most people will simply intuit the level of snake oil present in the preceding ad, but that still doesn't tell you what you need instead. It is one thing to say, "I don't need a $100-$500 magic mojo box," and another to know what you need. Turns out what you need is a transformer and not much else. You need to convert a low impedance, balanced signal from your audio interface into the kind of signal a guitar produces, which is unbalance, high impedance. Any passive direct box does this nicely. I mean ANY. Behringer makes a knockoff of the already cheap Whirlwind IMP that is more than good enough for this task.
The only thing you might not have lying around already is a cable that couples your audio interface with your direct box. My interface uses TRS outputs so I needed a cable with a TRS plug on one end, and a female XLR on the other (female b/c you are using the DI in the reverse signal direction from usual). I used parts left over from old projects, but you can easily track down a commercial cable with about any combination of terminals if you are too lazy to learn to solder or source parts.
The final piece of the puzzle is a really short 1/4" guitar cable. I use a cheap cable that came with a cheap pedal power supply. It has maybe an inch of cable. You want it short so it doesn't pick up noise.
Alternately you can use an inline transformer, which is built for exactly the type of thing we are doing. I think people usually use them to plug microphones into guitar and bass amps - now that I think about it I have one of these in a box somewhere and I got it for that purpose; I was trying to help a young sax player get a rig going so he could be heard with a band... Anyway this device solves two of your problems if you are starting from scratch - a short guitar cable (none) and the transformer. Plus its XLR connector is female, so if you have a cable that is almost right, that might help.
At this point I assume you have all the gear you need and you're ready to reamp a track in Reaper. Here are the steps.
At some point before I start any of this, I have guitar speaker cabinets in a closet with foam on every surface. I also have a stereo attenuator, so that I can record guitars at any volume I want (for ribbon and other sensitive mics) without having to hear the amp directly. You want to hear what the microphone hears.
Record your guitar tracks into your song. You need a clean, uneffected signal, so IMO the best way is to plug straight into your audio interface (assuming you have a hi-Z, unbalanced input). You can record the dry guitar signal and an amp at the same time, but this requires you to split the signal somewhere and that means there will be compromises. I look at the tracking stage as focused on the performance, while the editing and mixing stages are their own creative activities that require your full attention. So I use the SimulAnalog VSTs for a rough approximation of the sound I want and focus on giving a good performance.
Let's say I've recorded all My basic tracks and I'm ready to reamp the guitars. First I power off my left monitor.
Next I plug the left output from audio interface into my passive DI using the TRS/XLR female cable I described previously. I plug the "Inst High Z In" jack into the amp input jack with the shortest 1/4" instrument cable I own.
I mic the guitar amp speaker cabinet and plug the mic into input 1 (left) on my interface
In Reaper, I open the Routing Matrix (View > Routing Matrix) and reassign the master output to just the right speaker. I am using the left speaker for guitar signal now and I don't want anything else going out that side.
Create a new track for the reamped signal and call it Reamped
On the original track (with the clean, dry waveform), open the IO window (click on the IO icon just to the right of the track name).
Click Add a New Send and select the name new track called Reamped. It will probably be post-fader by default. Select Pre-Fader (Post-FX).
Volume fader on original track all the way down
Add the effect "ReaInsert". Hardware Sends, Left will be the left output on the audio interface. Hardware Returns, Left will bet the left input on the audio interface.
Make sure the amp is on and warmed up, and the mic has a little gain so it will get some signal. On the ReaInsert interface, click Auto Detect. You should hear a blip and see a number show up in the Samples box at the bottom. If not you did something wrong. This feature is the reason I do it this way - ReaInsert will figure out how much latency is in your setup and automatically adjust.
At this point you should have reamped audio coming in on the track called Reamped, and it should be in perfect sync with the song. You should have your speaker cabinet isolated somewhere where you can't hear it directly so that at this point you can fiddle with the amp and mic placement/selection to get a sound that jibes with the rest of the song.
- Go to the Reamped track and set it to record output. This is under the little button just left of the VU's. Click it and go to the Record:output submenu, then select Record:output (mono)
Now record arm the Reamped track
Hit the record button in reaper and let it run the length of the song. You now have a nicely reamped track.
Mute the original clean track and put it in a track folder of stuff too good to throw away.