How to Pick Music
I've gone over some general guidelines for choosing music before, but after having talked to a few people since then, it seems like it would be helpful to have more detailed instructions. I don't have the same resources as you, so I'll keep it broad, but hopefully this will still help.
Step One: Organize.
Use a spreadsheet program or a word processing program to set up a monthly music planner. Include columns for the date, the Entrance, Offertory, Communion, and Recessional (also a Meditation slot, if you want - I just put mine in the Communion cell). Put the month and year at the top, and include the year of the liturgical cycle you are on, if you are feeling fancy. I saved a blank one as a template, so that every time I want to make one, I can click "create new" and choose the template from the list and away we go. (I'd share my template with you except that I use a mac for my planning computer and a chromebook for my blogging and typesetting, so who am I to give organizational advice? But here's a .pdf of the one I made for my choir for this month, complete with typos.) Once you have done three years' worth of these, you can go back and check what you did for the fifth Sunday of Easter in year B last time - the readings haven't changed, why should the music?
Step Two: Pray.
You can use Come, Holy Spirit, the "Prayer before choosing music" that I put up here (down the page a bit), or something else - even just sit quietly in the presence of God for a moment.
Step Three: Read.
Read the Propers first - the Entrance, Offertory, and Communion antiphons. Then scan the Gospel. If those suggest anything straight off, pencil them in. Then read the Propers and Gospel for the previous and subsequent week. Is there a pattern? Sometimes the readings are linked to each other in weeks-long arcs, building up to something, such as the Eucharistic Dialogue in August from the sixth chapter of John. If you can, work to bring out the connection between weeks.
Step Four: Flip.
If you still have blank spots on your schedule, just pick up your hymnal and start paging through. Mix it up whether you start in the front or the back, or, if your resources are indexed by theme, start at a relevant section. The Worship 4 is organized by theme, but sometimes you need to get creative - for example, there's no "Mercy" or "Forgiveness" section, but there is a "Sacrament of Reconciliation" section that has some golden choices for Lent. But yes, literally just flip through and see what is there.
Step Five: Tweak.
Once you have everything filled, go back over it and wiggle it into place. Did you put four unison hymns on one Sunday? Boring! Are the styles of the music you chose too contrasting? Distracting! Are the hymns you chose for each Sunday well within the ability of your choir to brush up in a single practice? Or do you need to work your way up to something that you should place later in the month instead? Maybe you accidentally entered two hymns on the same Sunday that use the same tune? Or literally everything is arranged by Ralph Vaughan Williams? Well, maybe that last one isn't such a bad thing.
This is also the step where you open up your planner from last month and make sure there isn't too much repetition. Congregations like familiarity, but they also get bored. You will get more bored, more quickly, because you are paying more attention, plus singing them in rehearsal as well, and you are the one who really notices just how often you are singing Soul of My Saviour in a year.
General Notes on What Hymn Works Best Where:
The Entrance Antiphon is usually the one that seems the least related to the readings, and is set apart from them by the Penitential Rite and the Gloria. The words often focus on God's glory and goodness, suitable for a general hymn of praise. Themes vary between creation, mighty deeds, mercy, protection, etc.
Hot tip: In terms of length, the opening hymn will typically last two to three verses. If there is a concept or idea that needs more than this to complete - such as O God Our Help In Ages Past - the hymn is better off in another spot, unless you are certain that there will be incense used at the Entrance. (Sometimes you can get away with asking the altar server in charge of carrying the cross to walk a little more slowly if you just need a little extra time, but this tends to annoy the priests. Use your credit wisely in this area.)
The purpose of the Offertory hymn is to offer a space and a theme for meditation on the now-completed Liturgy of the Word, and to prepare the interior disposition of the congregation for the about-to-begin Liturgy of the Eucharist. Being the closest to the readings, it should reflect on or respond to what they say. At this time, the congregation does not have its attention drawn to other actions, so the words to the hymn can be tolerably complex.
Hot tip: The length of the offertory will vary depending on circumstances. At a normal Sunday Mass not during a pandemic, the offertory can be quite long, covering the collection, the presentation of the gifts, preparing the altar and chalice, and even occasionally incense; however, at a special liturgy (such as a funeral), there may be only the preparation of the gifts, in which case there is barely any time at all. Take the circumstances into account when planning, and ask before a special liturgy whether someone will be bringing up the gifts or whether the priest will be using incense at the offertory, as these are the biggest time variables.
The best Communion hymns are gentle and meditative. Some musicians like to favour hymns with a simple refrain so that the congregation may sing while in line for Communion without having to carry a book - or you can just sing Adoro Te Devote so many times that the entire congregation knows it by heart! Others prefer to take the opportunity to sing something beautiful that the congregation does not have to be involved in vocally, so that the faithful may turn their attention to interior preparation. Both approaches have their merits. If you aren't singing the Propers, and you can't find a hymn that goes with the text of the Communion antiphon, check the psalm versicles that go with it. If you still don't have anything, default to a Eucharistic hymn of your choice.
The recessional hymn is not technically part of the liturgy - the Mass ends when the congregation says “Thanks be to God.” Therefore you have the most latitude for stylistic and thematic choices with the recessional hymn. This is where you may honour a secular season (e.g. Fourth of July by singing America the Beautiful), and this is also the only musical “slot” in the liturgy where a Marian hymn is appropriate, outside of feasts of Our Lady. (During feasts of the Blessed Mother you will find that the Propers are often Marian antiphons, so, go to town!)
I hope you find this guide helpful! Did I leave anything out? Let me know in the comments section.