First, decide if you want to keep the seeds in or out of your relish. How hot do you like foods? Also jalapenos vary in heat. Taste a piece and decide. Maybe put some seeds in. Your call.
Chop your jalapenos fine and toss them with the minced garlic. Feel free to add a red pepper for a pop of color if you like (or to help cut the heat). In small mason jars (I use 4 oz) spoon in the chopped jalapenos until they just about reach the top. Don't press hard or "pack" them in, you need room for the liquid.
In a small pot, heat, vinegar, water, sugar, celery flakes and salt until it boils.
Remove from heat and allow it to sit for 2 minutes.
Pour liquid into your filled jars, allow it to cool down for 10 minutes or so, then screw the lids on, allow them to cool to room temperature and then place in fridge until you want to enjoy them.
Remember when working with jalapenos, you never know how hot or mild they may be. Be careful of your eyes, no rubbing your eyes for a while after working with them OR you will remember it for sure, lol!