Period play is when you and your partner engage in sexual acts while they are on their period and menstruating. It can be a great turn on for many people, as a few days into her period the woman's arousal reaches a peak, and so their desire for sex jumps up. The term 'period play' can cover a huge range of activities that you can do during this special time of the month.
There are some important things to remember when preparing for period play. First of all, you will want to check that your partner is on their period. Imagine your disappointment if you get the wrong date! Remember to discuss the idea with your partner - each person is different, and while some may love the idea, others might prefer to not do it. There could be many different reasons for this, so respect their wishes. You will need to keep in mind that period play can be messy. Old towels laid down on the bed are a good idea, or you could take your activities to the shower. Figure out between you which you'd prefer.
Another very important point is that, as you are playing with blood some safe sex practices will be required. Use any barriers you have correctly, and you may wish to consider things like gloved hands (such as surgical gloves) or a dental dam to minimise the risk.
When it comes to the actual period play, do whatever you both feel comfortable with. While one person may not enjoy oral sex while on their period, another might love it, so clarify this with your partner beforehand. Your partner may prefer some mutual masturbation or sex as opposed to oral. Whatever your partner's preferences, enjoy the moment and make sure you both have fun.