Hi Matthew,
Congrats on having what you believe was a good date with the woman you met on a dating site. That is never a guarantee. So, good for you.
Start your text with something that was shared between the both of you during that first date. Something fun that either puts a smile on her face or makes her chuckle.
This helps to keep your text lighthearted and fun. Then just ask if she would like to meet up on X day at X time for X activity.
That is really it.
Keep your text relatively short. Start with something lighthearted that references the first date so that she is hopefully reminded of how much fun she had with you, then offer the second date with a specific date, time and activity in mind.
Women like men who are decisive. If you spend too much time trying to butter her up through texting in the hopes of finally getting to a point where you ask her to a second date, you could easily turn her off. It can make you look like you lack confidence.
Keep your text short and decisive. If she truly enjoyed that first date, there is a good chance that she agrees to the second date.