Alice and Bob each create their own keypair. Bob creates the following transaction script:

IF
    <now + 3 months> CHECKLOCKTIMEVERIFY DROP
    <Bob`s pubkey> CHECKSIGVERIFY
ELSE
    <Alice`s pubkey> CHECKSIGVERIFY
ENDIF

Bob then hashes the script, encodes it as a P2SH address and sends the addresss to Alice. If Alice sends any Bitcoin to this address, she can redeem it with the following scriptSig:

0 <Alice`s signature> 0

After 3 months have passed, Bob can also redeem the transaction output Alice created with the following

0 <Bob`s signature> 1