Yup pullups (heaves) are great for lats.
Pronated grip (overhand) is best, with hands out wide - approx twice as wide as shoulder width.
That being said, variety is great so also using a narrow overhand grip and supinated (underhand) grip will help give a more complete back development.
Remember technique is the priority. 1 heave done well is worth 5 done poorly. Start with arms extended almost fully. They should be as straight as possible without locking them. Pull yourself up as far as you can - try to touch your chest to the bar. Lower yourself down again until arms are in original STRAIGHT starting position. Repeat.
Try to build up to ten of those consecutively. When you can do 2 or 3 sets of ten overhand heaves, you'll be doing well.
To increase weight (outside of a gym), just use your imagination. A backpack with rocks or some plastic bottles filled with water are great. Often people cross their legs and wedge a dumbell between their knees to add weight.
Good luck.
