What are the laws relating to terumah and maaser when I take two lemons from the tree and leave the rest to ripen?
You should bring the lemons home, and proceed to take terumah and maaser from the lemons.
This can be done with a berachah (the lemons are certainly tevel).
The following is the method by which the separation is made. Because the amount you separate is small (a little more than one hundredth of two lemons), and worth less than a perutah (a little over 10 agurot), you will need a perutah chamurah to complete the process – meaning, the coin you use (see below) must be a coin on which maaser has already been redeemed.
Order of Separation
1. Place all of the produce in front of you.
2. Remove slightly more than 1% of the produce. For example, if there are 100 oranges, one whole orange and a small part of a second orange are separated.
3. A coin, which is valid currency in the country in which the redemption is performed, is designated for redemption. At the time of this writing, one penny is sufficient.
4. Where it is certain that terumah and maaser was not separated, a blessing is recited.
5. Terumah and maaser may not be separated on Shabbos and Yom Tov.
6. It should be noted that while reciting this formula, neither the produce nor the separated portion should be moved, since the formula refers to designated locations.
7. The following text is recited:
“The amount in the northernmost part of the separated portion which is greater than 1% of the total amount of the produce shall be terumah gedolah.
“The remaining part of the separated portion, plus an additional 9% on the northernmost side of the produce shall be maaser rishon.
“The part of the separated portion that was previously designated maaser rishon shall be terumat maaser.
“10% of the remaining produce in the southern side shall be either maaser sheni or maaser ani, in accordance with the year of the shemittah cycle during which the produce was grown .
“If the 10% on the southern side is maaser sheni, it shall be redeemed and its kedushah, calculated at its value plus 25%, transferred to a perutah within the designated coin. If the produce is neta revai, it shall be redeemed and its kedushah, calculated at its value plus 25%, transferred to a perutah of the designated coin.”
Alternatively, if one has difficulty with the full text or it is not available, this simplified text may be recited: “All separations of terumah and maaser and redemptions of maaser sheni and neta revai shall be effected in accordance with the text of the Chazon Ish, and all relevant redemption applied to a perutah of the designated coin.”
8. The separated produce (the part which was more than l%), and the designated coin are wrapped and discarded.