In this article we will look at the readiness to be done during cross forest migration from exchange 2010 to 2013.

There are multiple ways to perform this and this is again one of the best practices that can be followed.
The first and the foremost thing is that we need to ensure that DNS is setup properly vice versa between the source and the target forest.

Steps to ensure for DNS setup:
1) Check if you have the same root DNS for both of the forest DNS namespaces. Make sure that the root zone contains delegations for each of the DNS namespaces.
Also, update the root hints of all DNS servers.
To Update root hints on the DNS server follow this article –  http://go.microsoft.com/fwlink/?LinkId=92717
If there is no shared root DNS server for both of the forests and the root DNS servers for each forest DNS namespace are running a Windows Server operating system, configure DNS conditional forwarders in each DNS namespace to route queries for names in the other namespace.
To Configure DNS server forwarders follow this article http://go.microsoft.com/fwlink/?LinkId=92718
IMP :  If there is no shared root DNS server and the root DNS servers for each forest DNS namespace are not running a Windows Server operating system, configure DNS secondary zones as well in each DNS namespace to route queries for names in the other namespace.
To Add a secondary server for an existing zone follow this article http://go.microsoft.com/fwlink/?LinkId=92719

After above steps are done validate the DNS configuration through nslookup . You can follow the below article as well if you are having doubt in verifying through NSLOOKUP

https://technet.microsoft.com/en-us/library/977fa8ed-ec71-4d39-9f9e-9facd5a61364
Create a new forest trust:

2. Create a forest trust
a. Use account which belongs to Domain Admins or Enterprise Admins of domain. Open Active Directory Domains and Trusts on a DC of domain. To open Active Directory Domains and Trusts, click Start , click Administrative Tools , and then click Active Directory Domains and Trusts .
To open Active Directory Domains and Trusts in Windows Server® 2012, click Start , type domain.msc .
b. In the console tree, right-click the domain that you want to administer, and then click Properties .
c. On the Trusts tab, click New trust , and then click Next .
d. On the Trust Name page, type the Domain Name System (DNS) name (or NetBIOS name) of the domain, and then click Next .
e. On the Trust Type page, click Forest trust , and then click Next .
f. On the Direction of Trust page, do one of the following:
o To create a two-way, forest trust, click Two-way .
Users in this forest and users in the specified forest will be able to access resources in either forest.
o To create a one-way, outgoing forest trust, click One-way:outgoing .
Users in this forest will not be able to access any resources in the specified forest.
g. Continue to follow the instructions in the wizard.

Validate the created trust.

Once the above is completed you can run prepare new request by following the below article

https://technet.microsoft.com/en-us/library/ee861103%28v=exchg.150%29.aspx

Once prepare move request is completed run the new move request

Also set the move request large item limit to minimum to 50 so that large mailboxes move will not create an issue during migration by running the below command

Get-MoveRequest | Set-MoveRequest -LargeItemLimit 50
Rollback Plan if in case of anything went wrong:

Delete trust accordingly. To perform this procedure, you must be a member of the Domain Admins group or the Enterprise Admins group in Active Directory Domain Services (AD DS), or you must have been delegated the appropriate authority.
To remove a trust using the Windows interface
1. Open Active Directory Domains and Trusts. To open Active Directory Domains and Trusts, click Start , click Administrative Tools , and then click Active Directory Domains and Trusts .
To open Active Directory Domains and Trusts in Windows Server® 2012, click Start , type domain.msc .
2. In the console tree, right-click the domain that contains the trust that you want to remove, and then click Properties .
3. On the Trusts tab, under either Domains trusted by this domain (outgoing trusts) or Domains that trust this domain (incoming trusts) , click the trust to be removed, and then click Remove .
4. Do one of the following, and then click OK :
o Click No, remove the trust from the local domain only .
If you select this option, we recommend that you repeat this procedure for the reciprocal domain.
o Click Yes, remove the trust from both the local domain and the other domain .
If you select this option, you must type a user account and password with administrative credentials for the reciprocal domain.

Also you can follow the below excellent write up about cross forest migration by exchange server MVP Prabhat Nigam

http://msexchangeguru.com/2013/11/03/e2013crossforestmigration/

Thanks & Regards

Sathish Veerapandian

MVP – Exchange Server