I would advise you to never work for any of them.
But if you must, then at least avoid Labour Ready, they are the worst by far, in my opinion. Diversified seems pretty terrible to me as well, but it is possible to get decent jobs through them, though I doubt it happens often.
All the other offices I know are not as terrible.
Most of the agencies in the city are downtown, if you start early, you can hit them up in a day. Here's what you can expect:
When you walk in and ask to sign up, you will be required to fill out some simple forms that will require your basic info, and for you to list your skills, limitations, and answer some skill testing questions. You will also have to do a WHMIS test, and occasionally, Fall Arrest or stuff like that as well.
If you've seen the movie Idiocracy, the testing portion of the process will be very familiar to you.
They will also want to see your SIN card and a piece of photo ID.
Now, if you're in an office geared towards day labour (Labour Ready, Diversified, Pro Temps), you're pretty much done at this point. They will put all your forms in your file and tell you to come in the morning.
If you're at an office geared to longer term assignments (ManPower, Kelly, Randstad) you will also have a short interview with someone. And may also have to do some additional skill tests related to whatever jobs they have available (if any).
These offices will not require you to come in every day in the hopes of getting work, they will simply put your info in a file and call you if they have anything for you. But if you don't keep bugging them, they will most likely forget about you.
If you get called for work, make sure you have PPE, not just what they tell you to bring, but everything, (especially dust masks, if you're sensitive to that) since sometimes the conditions of the job will be different than what you were told.
Most places will give you forms that need to be filled out by the supervisor at the job site. These forms track your hours and are what make sure you get paid, so don't lose them. You will most likely be paid daily or weekly, depending on the office.
If they pay daily, just bring in your form and cash out. If weekly, you can set up direct deposit or just get a cheque every week.
If you're working for a place that pays weekly, and you need an advance, some places will allow you to... buy your money.
When I worked for Pro Temps (which was a long time ago now, so my information is not up to date), you got paid every Friday. But if you were desperate, every Wednesday you could pay them to give you an advance. If I remember right, the fee was $20 for the first $50, and $10 for every subsequent $50.
I hope you're not so bad with your money, or in such a desperate situation that you need to do this, I just wanted to highlight something that would give you a small idea of what kind of places these agencies are.
If you have to do this, keep your head high, stay safe, and at the first available opportunity, GET OUT.