If you have anything resembling a stereo system from bygone days, there should be a spare "tape out" or similar set of LR (red/white) RCA jacks. Get a stereo RCA to 1/8" (3.5mm) stereo cable (male RCAs - male 1/8"). Go from your stereo to the Zoom H2n LINE IN jack. Set the Zoom to record WAV, 44.1kHz/24-bit, and set the recording level so the audio from the stereo does not clip, i.e., the peak levels you can see are at least 3dB below 0dB and the clipping indicator NEVER lights up.
Load up a good sized SD card, maybe have a couple around. Record your 45s and LPs. You can stop/start between songs on the LPs, but I found it easier to just do a full side. Make sure your phono cartridge and turntable are in good shape, running at the right speed, etc.
This is very time consuming because it happens at "real time" and not like ripping a CD. You might want to be very particular about what you convert. I quickly decided that the only thing worth doing this for were a few LPs that have never been released on CD that meant something to me. Anything else, I scoured eBay, et al, to find good CDs. (My ears really cannot tell the difference at this point, and the good stereo system went to the daughter years ago.)
For starters, you can use Audacity (freeware/open-source audio editing app). Pop the SD out of the H2n and into your computer, or connect the H2n via USB cable and drag the recorded audio files to your computer. Label the originals and save them (something I did not do, and kicks myself for probably once a year).
Now, you can do the following steps in Audacity or GarageBand, but basically you'll want to open the original file up, and then export individual songs that have been normalized, possibly EQ'd and had noise reduction done, as necessary. If you export individual songs in any format other than the original, e.g.., MP3/AAC or CD format (44.1kHz/16-bit), you should probably dither, and I assume GB does that semi-automatically, but in Audacity you'll want to set it to "Shaped" in the Quality (Preferences) setting.