I used a regular soldering iron (not adjustable for heat settings) on a stock Glock grip. I was happy with the final result, but there was a learning curve to it. I had only 1 tip profile and heat settings. To control heat, I had to control how long I held the tip to the polymer. Too much heat for too long and you could pull up molten plastic filaments or just create an ugly hole that's too deep. To control shapes, I had to hold the iron at different angles when pressing it into the frame, or use techniques like touching the iron to the plastic and dragging it across.
I'd say that if you're not just going with what you got like I did, that you at least buy a soldering iron with adjustable heat settings. Maybe look into a wood burning kit. Definitely practice on some scrap polymer first. Maybe buy a cheap accessory like a pistol grip or vertical foregrip first and burn that up so you get a feel for it.