DP and DPP Preparations

Before you go on doing a DP (addressing you as the bf of the girl, or the girl herself), you should make sure you've done the following first:

Dildo DP

Try cock in ass and dildo in pussy. If that works well, it's a good sign. If not, keep practicing. Avoid using a hard dildo if you can: hard dildos are very unlike cocks, which are flexible (even when hard) and can bend and compress.

For DPP: same as above, except you stick in both cock and dildo in pussy. Easiest way is probably to first insert dildo into pussy, then cock above dildo, missionary style. That way, the guy can easily hold the dildo while fucking, preventing it from sliding out.

Buy lubricant

It's always a good idea to use lube during anal sex, more so during DP. Same goes for DPP, use lube, as it will be tight inside pussy!

Before the actual DP or DPP, lube both outside and inside of pussy, as well as both cocks (for condom users: lube the outside obviously, the inside should not be lubed as it may slide of the cocks then).