When I had a regular day job with 26 biweekly paychecks, I divided the total allowed annual Roth IRA contribution evenly across all paychecks. For someone who gets paid on the same schedule and wants to contribute the maximum $6,000 per year (the limit for 2019 and 2020), that would be $230.76 per paycheck (plus 24 cents one time during the year).
You can contribute at any time, however. As someone who is now self-employed, I make a lump contribution for the entire balance once per year. You can divide it up any way you choose. But remember that investing in your Roth IRA (or traditional IRA) on a regular basis is an important part of saving for retirement.
Contributions are allowed into the following year until tax day. For 2019, for example, you can make contributions through April 15, 2020. But you have to get the contribution in before you file your tax return.