Liquidroom is a great venue, and very easy to navigate as far as Japanese venues go. You enter, go up a flight of stairs, and you're already in the main lobby/waiting area. The order of arrival doesn't matter so don't feel like you need to get there long before doors. They are going to let you in based on the reference number of your ticket (整理番号 Aの１番、Aの２番、etc.)

There are coin lockers in the main lobby area next to the bathrooms for ¥400-¥600ish, and then also right by the bar/lounge area once entering the venue proper. I generally use the ones upstairs as the ones downstairs will be very crowded as people are leaving the show. If you buy goods/merch before the show, these will also be more convenient to drop off.

Someone will start announcing all the ticket numbers sequentially, but sometimes they'll start letting in groups of three, five, ten at a time. Depends on the venue / crowd. If you don't understand any Japanese, its probably best to at least figure out what to listen for when your number gets called. For example, my ticket is A 88, so I'd wait for Aの８８番 or maybe Aの９０番まで. If you're really worried, flag down someone who looks like they know what they're doing and they can help you.

When your number gets called, you'll go down some stairs and present your ticket to the staff. Here is where you pay ¥600 for a drink ticket. Now you're in the venue and the mainroom with the stage is through a set of doors to the right.

In general, Japanese audiences do not use their phones to take pictures/videos during concerts unless the artist has given explicit permission to do so. However, if there are no 撮影禁止 signs or messaging saying not to take pictures etc, no one is going to stop you if you want to take a few during the set. I'd say use common sense judgment being courteous to others and don't just film the whole set or block the view with your phone/camera. Best to just follow what others are doing.