best methods to start with fertilization

You're looking for a third opinion? What exactly are you growing in? If you're taking the minerals the plant needs out by using demineralized water, what's your plan to provide them some other way, or do you have one? Last grow, which is curing now (my tent is now just waiting for the bean to sprout in its paper towel before it gets planted), I used Pro-mix "Premium Potting Soil" with plain de-chlorinated tap water for 21 days before adding anything, and the only stress to the plant during that time was of the type I created through making some early mistakes early, though the grow turned out well in terms of pure yield per watt used relative to my running it with almost no nutes (as close as I'll ever come). This grow, I plan to add a kelp based phytoplankton soil conditioner with an NPK value of 4-0-1 as my main veg nute (I'm only using it for veg) at 1/4 strength on day 7, half strength on day 14, and full strength on day 21 which is where the strain I grow begins transitioning to pre-flower, so I want to experiment with adding a little more than it strictly needs because in looking at other grow diaries, it seems like this strain might be okay with it and I'll be looking for any signs of nitrogen toxicity and I'll be prepared to flush and adjust if I'm wrong there (and I'm more familiar with it now that I've grown it). If you're three weeks in, and if that's an auto, you're probably close to early flower now and your plant should be ready for transition nutes and then bloom nutes. "Transition" nutes just reflect that because of the pre-flower stretch that's coming, you'll still need a lot of nitrogen and more then you'll find in a pure bloom nute as after the pre-flower stretch (after week 4 or 5), nitrogen needs taper down pretty quickly while there's an intermediate need in weeks 4-5 for more phosphorous, and past week 3 it'll also begin needing more and more potassium: the farther you go in flower, the greater the potassium need while the need for the other 2 falls off over time. This is the big three NPK ratio, and you need enough of each, and at the right time, plus calcium, magnesium, and several other trace elements. Now is the time to pick a nutrient schedule and brand and start feeding it, and also a good time to begin bookmarking resources on nutrient deficiencies/toxicity so that you know what to look for so you can catch either early. These nutes can be in liquid or powder form, and some are used only once and may be added into the soil directly as "top dressing," or mixed in periodically (weekly is normal for most nutes) with your watering, except for the final week or 2 where you only give plain water to flush them out.