You should get a testing kit, at minimum test strips, everyone says just buy the master test kit because you will use it for ever and it gives more accurate readings.
Depending on your readings would be how much water to change, lower levels (safe zone) of bad stuff less water 25%, higher dangerous levels more 50%, also maybe change 2x, like am/pm in same day if you have dangerous levels.
Do you have any friends with healthy tanks and a good used filter, or you can piggyback a clean new filter behind their old one to get some good bacteria.
Just do a search here for humane fish in cycle, or fish in cycle (or cycling), at least one of the guys on here has a step by step.